Make RequirementSummary equality and hash unsentive to version format variation

[vphilippon]

Fixes #1192

packaging.specifiers.SpecifierSet (ireq.specifier) provides equality and
hash definition that are based on a canonical specifier representation.

Basing RequirementSummary equality and hash on those allows to easily
side-step any version/specifier normalization concerns on our side.

Effectively, makes equality and hash of
test_package==1.2.0 and test_package==1.2 to be the same, as expected.

Changelog-friendly one-liner: Fix pip 20.2 compatibility issue that caused pip-tools to sometime fail to stabilize in a constant number of rounds.

Contributor checklist

• Provided the tests for the changes.
• Gave a clear one-line description in the PR (that the maintainers can add to CHANGELOG.md on release).
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[vphilippon]

Argh, forgot to check with pip<20.2 before submitting 🤦‍♂️ .
I'll look if there's a way to easily fix that across the board.

Alternatively, we can drop the older pip versions as per our new policy, right?

[vphilippon]

I locally tested, and the new code actually works with pip 20.0, 20.1 and 20.2.

But only pip 20.2 has the updated vendored packaging version such that the canonical SpecifierSet is used for equality and hashing.
But it's also only with pip 20.2 that pip-tools needs that enhanced equality/hash on RequirementsSummary.

So I should be able to put a condition mark on some specific test case, and this should be good.

[atugushev]

Awesome 🎉 I've tested with this reproducer locally — it works.

[atugushev]

@vphilippon

Alternatively, we can drop the older pip versions as per our new policy, right?

Yeah, there is a #1191 to drop pip-20.0.

[atugushev]

Thanks @vphilippon!