version: fix allows for local and post releases according to PEP 440

[radoering]

Resolves: python-poetry/poetry#4729

Partially overlaps with #379 but only considers simple version (not ranges).

• Added tests for changed code.
• Updated documentation for changed code.

According to PEP 440:

For example, given the version 1.1.post1, the following clauses would match or not as shown:

== 1.1 # Not equal, so 1.1.post1 does not match clause
== 1.1.post1 # Equal, so 1.1.post1 matches clause
== 1.1.* # Same prefix, so 1.1.post1 matches clause

and

If the specified version identifier is a public version identifier (no local version label), then the local version label of any candidate versions MUST be ignored when matching versions.

If the specified version identifier is a local version identifier, then the local version labels of candidate versions MUST be considered when matching versions, with the public version identifier being matched as described above, and the local version label being checked for equivalence using a strict string equality comparison.

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