Fix use of URLValidator to correctly validate RELEASE_CHECK_URL

[jathanism]

Fixes: #5977

This is a minor patch that makes the test in netbox.settings to correctly validate that RELEASE_CHECK_URL is a valid URL. I wasn't able to find a place where these settings are tested in the test suite, so I am providing some interactive Python shell output to illustrate the fix:

In [1]: from django.core.validators import URLValidator

In [2]: from django.core.exceptions import ImproperlyConfigured, ValidationError

In [3]: RELEASE_CHECK_URL = 'bogus url'

In [4]: if RELEASE_CHECK_URL:
   ...:     validator = URLValidator(
   ...:         message=(
   ...:             "RELEASE_CHECK_URL must be a valid API URL. Example: "
   ...:             "https://api.github.com/repos/netbox-community/netbox"
   ...:         )
   ...:     )
   ...:     try:
   ...:         validator(RELEASE_CHECK_URL)
   ...:     except ValidationError as err:
   ...:         raise ImproperlyConfigured(str(err))
   ...:
---------------------------------------------------------------------------
ValidationError                           Traceback (most recent call last)
<ipython-input-4-8bf07c825907> in <module>
      8     try:
----> 9         validator(RELEASE_CHECK_URL)
     10     except ValidationError as err:

~/Library/Caches/pypoetry/virtualenvs/nautobot-b2ttWva6-py3.8/lib/python3.8/site-packages/django/core/validators.py in __call__(self, value)
    104         if scheme not in self.schemes:
--> 105             raise ValidationError(self.message, code=self.code)
    106

ValidationError: ['RELEASE_CHECK_URL must be a valid API URL. Example: https://api.github.com/repos/netbox-community/netbox']

During handling of the above exception, another exception occurred:

ImproperlyConfigured                      Traceback (most recent call last)
<ipython-input-4-8bf07c825907> in <module>
      9         validator(RELEASE_CHECK_URL)
     10     except ValidationError as err:
---> 11         raise ImproperlyConfigured(str(err))
     12

ImproperlyConfigured: ['RELEASE_CHECK_URL must be a valid API URL. Example: https://api.github.com/repos/netbox-community/netbox']

[jeremystretch]

Thanks for the PR, but in the future please wait for the issue to be accepted per our contributing policy.