Fix url encoding of strings with slashes in URLs (#3263)

Fixes https://github.com/PyGithub/PyGithub/issues/3262.

This changes the URL encoding function used for strings in URLs.
Currently, this uses `urllib.parse.quote`, which makes an exclusion of
URL-encoding the `/` character:
> urllib.parse.quote(string, safe='/', encoding=None, errors=None)
Replace special characters in string using the %xx escape. Letters,
digits, and the characters '_.-~' are never quoted. By default, this
function is intended for quoting the path section of a URL. The optional
safe parameter specifies additional ASCII characters that should not be
quoted — its default value is '/'.

The GitHub APIs expect that `/` _should_ be URL encoded. For example, if
my environment name contains `/` I get a 404 from the GitHub API.

I have modified the call to remove this exclusion for the `/` character,
which causes the environment name to be correctly encoded. I have
updated the test cases to provide coverage for this change.

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Co-authored-by: Enrico Minack <github@enrico.minack.dev>