Update assertContains and assertNotContains signatures

[SebCorbin]

Any text passed to assertContains and assertNotContains will be casted with str, see https://github.com/django/django/blob/a0323a0c44135c28134672e6e633e5f4a7a68d5d/django/test/testcases.py#L580-L581

[sobolevn]

Hi, looks like your IDE is doing something strange:

django-test-migrations (https://github.com/wemake-services/django-test-migrations)
+ /home/runner/work/django-stubs/django-stubs/django-stubs_to_test/django-stubs/test/testcases.pyi:88: error: Invalid non-printable character U+00A0  [syntax]

Please, fix it :)

[SebCorbin]

Ha, I used github web interface so I may have unintentionally entered a NBSP.

[sobolevn]

Thanks!