fix(markers): only parse versions on certain keys

[JP-Ellis]

According to the environment markers, most markers are strings, with only a small subset being use to handle versions. As such, this PR changes the behaviour to use version comparison only on those keys which are dealing with versions.

Note that the choice of only doing version comparison on keys which are defined as being versions results in one failure for a custom key:

    def test_prefers_pep440(self):
>       assert Marker('"2.7.9" < "foo"').evaluate(dict(foo="2.7.10"))
E       assert False
E        +  where False = evaluate({'foo': '2.7.10'})
E        +    where evaluate = <Marker('"2.7.9" < "foo"')>.evaluate
E        +      where <Marker('"2.7.9" < "foo"')> = Marker('"2.7.9" < "foo"')
E        +    and   {'foo': '2.7.10'} = dict(foo='2.7.10')

From my reading of the specification, it is unclear whether the use of version comparison in this case is meant to be supported or not.

I can see a few ways of moving forward

1. If version comparisons are only meant to be used with the defined set of keys, then this PR is ready to go, requiring only that the one test be changed to expect failure.

2. If version comparison should be the default, and only some keys are exempted from the version comparison, then we can invert the logic in _eval_op from an if key in MARKERS_REQUIRING_VERSION: to an if key in MARKERS_REQUIRING_STRING_COMPARISON. If this is the case, then we would need to define what those keys are.

3. An intermediate approach would be to inspect the operator and, for example, using version comparison for <, <=, >=, >, ~= and ===, and string comparison for == and !=. I don't like this approach as it muddies the logic, and I think it would be better to be explicit about which keys are to be treated as versions.

Resolves: #938
Replaces: #932 (this is a different approach)

[Liam-DeVoe]

Replaces: #932 (this is a different approach)

I don't think this supersedes #932, because it's still possible to hit InvalidVersion with manual use of SpecifierSet (eg the example in #767). FWIW, the issue in this pull would actually be fixed by #932, though I think the explicit specification of markers which require a version here is good to have regardless.

[henryiii]

I think the spec needs to change. It states the type depends on the operator used, not on that table (not sure why that table is there, then!), and == is one of the version types. I've proposed an spec update in https://discuss.python.org/t/100287 to allow this to move forward (with platform_release added to the "version-like" set for now, to avoid any real-world effect).

[henryiii]

I've updated this to follow the spec change update in pypa/packaging.python.org#1971.