In Python 3.11 the standard library `cgi` module was deprecated with a
planned removal set for Python 3.13. In preparation for that removal,
this commit removes the usage of this deprecated module and replaces it
with the still supported standard library `email` module which is what
the documentation points to as an alternative for how `cgi` was
previously used. This should still be compatible with all the supported
Python versions but will be more future proof and not emit any
deprecation warnings with Python 3.11 anymore.

We can do this ourselves quite easily.

Signed-off-by: Stephen Finucane <stephen@that.guru>

This is kept separately from the reintroduction of 'try_imports' to keep
that patch reasonable.

Signed-off-by: Stephen Finucane <stephen@that.guru>

There are few if any people still (knowingly) using the 'extras' lib.
OpenStack users have their own implementation (sigh) in 'oslo.utils' and
everyone else is getting by with try-except statements. Reintroduce this
utility to 'testtools.helpers', allowing us to eventually drop the
dependency on extras. Note however that we can't actually do this yet
since 'fixtures' has an implicit dependency on 'extras' through
'testtools', and we break that by removing it here.

Signed-off-by: Stephen Finucane <stephen@that.guru>

This was being used in places to handle differences between Python 2 and
Python 3. Since we only care about the latter now, the utility can be
removed.

Signed-off-by: Stephen Finucane <stephen@that.guru>

This was used to workaround some issues with 'hasattr' on Python 2.x
versions, discussed in the source for extras (where this utility was
moved to make it more useful to others) [1] and elsewhere on the
internet [2]. These issues are no longer present on the versions of
Python we now support (Python 3.6+) so this is no longer necessary.

[1] https://github.com/testing-cabal/extras/blob/1.0.0/extras/__init__.py#L101-L108
[2] https://hynek.me/articles/hasattr/

Signed-off-by: Stephen Finucane <stephen@that.guru>