Bytes object is not JSON serializable when updating my python environment

I developed an app in Flask that worked. I am trying to upgrade my environment from 2.7 to 3.6. When I hit the endpoint calling the form, it generates the following error:

Traceback (most recent call last):
  File "/Users/Melvyn/Documents/personnel/python/TheSwitchWebApp/venv/lib/python3.6/site-packages/flask/app.py", line 1997, in __call__
    return self.wsgi_app(environ, start_response)
  File "/Users/Melvyn/Documents/personnel/python/TheSwitchWebApp/venv/lib/python3.6/site-packages/flask/app.py", line 1985, in wsgi_app
    response = self.handle_exception(e)
  File "/Users/Melvyn/Documents/personnel/python/TheSwitchWebApp/venv/lib/python3.6/site-packages/flask/app.py", line 1540, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/Users/Melvyn/Documents/personnel/python/TheSwitchWebApp/venv/lib/python3.6/site-packages/flask/_compat.py", line 33, in reraise
    raise value
  File "/Users/Melvyn/Documents/personnel/python/TheSwitchWebApp/venv/lib/python3.6/site-packages/flask/app.py", line 1982, in wsgi_app
    response = self.full_dispatch_request()
  File "/Users/Melvyn/Documents/personnel/python/TheSwitchWebApp/venv/lib/python3.6/site-packages/flask/app.py", line 1614, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/Users/Melvyn/Documents/personnel/python/TheSwitchWebApp/venv/lib/python3.6/site-packages/flask/app.py", line 1517, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/Users/Melvyn/Documents/personnel/python/TheSwitchWebApp/venv/lib/python3.6/site-packages/flask/_compat.py", line 33, in reraise
    raise value
  File "/Users/Melvyn/Documents/personnel/python/TheSwitchWebApp/venv/lib/python3.6/site-packages/flask/app.py", line 1612, in full_dispatch_request
    rv = self.dispatch_request()
  File "/Users/Melvyn/Documents/personnel/python/TheSwitchWebApp/venv/lib/python3.6/site-packages/flask/app.py", line 1598, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/Users/Melvyn/Documents/personnel/python/TheSwitchWebApp/TheSwitchWebApp/controllers/account/register.py", line 15, in display_signup_form
    form = RegisterForm(request.form)
  File "/Users/Melvyn/Documents/personnel/python/TheSwitchWebApp/venv/lib/python3.6/site-packages/wtforms/form.py", line 212, in __call__
    return type.__call__(cls, *args, **kwargs)
  File "/Users/Melvyn/Documents/personnel/python/TheSwitchWebApp/venv/lib/python3.6/site-packages/flask_wtf/form.py", line 88, in __init__
    super(FlaskForm, self).__init__(formdata=formdata, **kwargs)
  File "/Users/Melvyn/Documents/personnel/python/TheSwitchWebApp/venv/lib/python3.6/site-packages/wtforms/form.py", line 278, in __init__
    self.process(formdata, obj, data=data, **kwargs)
  File "/Users/Melvyn/Documents/personnel/python/TheSwitchWebApp/venv/lib/python3.6/site-packages/wtforms/form.py", line 132, in process
    field.process(formdata)
  File "/Users/Melvyn/Documents/personnel/python/TheSwitchWebApp/venv/lib/python3.6/site-packages/wtforms/csrf/core.py", line 43, in process
    self.current_token = self.csrf_impl.generate_csrf_token(self)
  File "/Users/Melvyn/Documents/personnel/python/TheSwitchWebApp/venv/lib/python3.6/site-packages/flask_wtf/csrf.py", line 134, in generate_csrf_token
    token_key=self.meta.csrf_field_name
  File "/Users/Melvyn/Documents/personnel/python/TheSwitchWebApp/venv/lib/python3.6/site-packages/flask_wtf/csrf.py", line 47, in generate_csrf
    setattr(g, field_name, s.dumps(session[field_name]))
  File "/Users/Melvyn/Documents/personnel/python/TheSwitchWebApp/venv/lib/python3.6/site-packages/itsdangerous.py", line 565, in dumps
    payload = want_bytes(self.dump_payload(obj))
  File "/Users/Melvyn/Documents/personnel/python/TheSwitchWebApp/venv/lib/python3.6/site-packages/itsdangerous.py", line 847, in dump_payload
    json = super(URLSafeSerializerMixin, self).dump_payload(obj)
  File "/Users/Melvyn/Documents/personnel/python/TheSwitchWebApp/venv/lib/python3.6/site-packages/itsdangerous.py", line 550, in dump_payload
    return want_bytes(self.serializer.dumps(obj))
  File "/Users/Melvyn/Documents/personnel/python/TheSwitchWebApp/venv/lib/python3.6/site-packages/itsdangerous.py", line 51, in dumps
    return json.dumps(obj, separators=(',', ':'))
  File "/usr/local/Cellar/python3/3.6.1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/json/__init__.py", line 238, in dumps
    **kw).encode(obj)
  File "/usr/local/Cellar/python3/3.6.1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/json/encoder.py", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/usr/local/Cellar/python3/3.6.1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/json/encoder.py", line 257, in iterencode
    return _iterencode(o, 0)
  File "/usr/local/Cellar/python3/3.6.1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/json/encoder.py", line 180, in default
    o.__class__.__name__)
TypeError: Object of type 'bytes' is not JSON serializable

      

        
        
        
      

    

It generates this error when I hit the following endpoint:

@register_endpoint.route('/', methods=['GET'])
def display_signup_form():
    error = None
    form = RegisterForm(request.form)
    return render_template("auth/register.html", form=form, error=error)

      

        
        
        
      

    

The error seems to be coming from the form:

class RegisterForm(Form):
    first_name = StringField('First name', [InputRequired(message='First name is mandatory')])
    email = StringField('Email Address', [Email(), InputRequired(message='Forgot your email address?')])

      

        
        
        
      

    

I am not getting the source of the error. In this piece of code, I am not trying to serialize anything to JSON. Where does the error come from?