Fully cover handling arbitrary equality (`===`) in `Specifier` and `SpecifierSet`.

[notatallshaw]

As part of implementing #940 I am finding a lot of gaps in SpecifierSet for arbitrary equality.

For example are any of these valid SpecifierSets:

• ===1.0
• ===1.0,===1.0
• ===1.0,===1.0.0
• >=1.0,===1.0.0
• ===foo
• >=1.0,===foo

The problem is _coerce_version has to be used as part of filter to properly implement PEP 440, but it will break if arbitrary strings are used validly as part of ===foo.

I believe the first step is to accept #932, and then add many edge cases tests for arbitrary equality and review if their output looks reasonable or not.

I am not yet working on a PR for this, I will wait until #932 is accepted or rejected.

[notatallshaw]

Here's are some specific issues:

=== works are expected when filtering against one specifier:

>>> list(SpecifierSet("===1.0").filter(["1.0"]))
['1.0']
>>> list(SpecifierSet("===1.0").filter(["1.0.0"]))
[]
>>> list(SpecifierSet("===1.0.0").filter(["1.0"]))
[]
>>> list(SpecifierSet("===1.0.0").filter(["1.0.0"]))
['1.0.0']

But breaks when filtering against two specifiers:

>>> list(SpecifierSet("===1.0,===1.0.0").filter(["1.0"]))
['1.0']

Further, arbitrary versions do not work as per PEP 440:

>>> list(SpecifierSet("===foo").filter(["foo"]))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/damian/opensource/contributions/packaging/src/packaging/specifiers.py", line 945, in filter
    if prereleases is None and self.prereleases is not None:
  File "/home/damian/opensource/contributions/packaging/src/packaging/specifiers.py", line 727, in prereleases
    if any(s.prereleases for s in self._specs):
  File "/home/damian/opensource/contributions/packaging/src/packaging/specifiers.py", line 727, in <genexpr>
    if any(s.prereleases for s in self._specs):
  File "/home/damian/opensource/contributions/packaging/src/packaging/specifiers.py", line 265, in prereleases
    if Version(version).is_prerelease:
  File "/home/damian/opensource/contributions/packaging/src/packaging/version.py", line 203, in __init__
    raise InvalidVersion(f"Invalid version: {version!r}")
packaging.version.InvalidVersion: Invalid version: 'foo'