SpecifierSet.filter should allow pre-release when no final version matches specifiers, same as Specifier.filter

[notatallshaw]

Fixes #856

This brings SpecifierSet in-line with Specifier and the spec. I believe this behavior was incorrectly created from #29, which determined ahead of filtering whether to allow pre-releases or not, but did not introduce a fallback when only pre-releases matched the specifiers.

Interestingly, even though the old behavior is listed in the doc string there were previously no tests for this behavior:

>>> list(SpecifierSet(">=1.2.3").filter(["1.2", "1.5a1"]))
[]

The docstring has been updated and the equivalent tests are added.

[notatallshaw]

FWIW, I prefer writing a method to return early, and not have large else blocks, but this PR focuses on fixing the bug without refactoring or writing code that conflicts with the existing style.

[notatallshaw]

I've updated the tests of both Specifier.filter and SpecifierSet.filter so they match each other, with the exception of the empty string specifier which SpecifierSet accepts but Specifier doesn't.

[notatallshaw]

This is ready for review / merge. FYI the relevant line in the spec is:

accept remotely available pre-releases for version specifiers where there is no final or post release that satisfies the version specifier

Specifier.filter already conforms, but SpecifierSet.filter does not, which means dependency resolution tools like pip that rely on packaging do not correctly follow the spec here.