Update type hints of PEP440Version class

[NiklasRosenstein]

This change allows Mypy to infer that methods like parse() or replace() on a subclass of PEP440Version return the same type and not falling back to the PEP440Version base type.

Example:

from poetry.core.semver.version import Version

version = Version.parse('1.0.0')
next_version = version.next_major()

reveal_type(version)  # poetry.core.semver.version.Version
reveal_type(next_version)  # poetry.core.semver.version.Version  (used to be PEP440Version)

Note that poetry-core can only be used with Mypy after merging #269 anyway.

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[danieleades]

this seems slightly surprising to me. I guess i've not seen this construction before. I would have expected to add typing appropriate to the derived classes by specialising the derived class signatures, not the base class signatures. adding type annotations to self and cls is a little jarring

[dimbleby]

Pretty annoying to have to implement derived methods just for the sake of the typing, though?

The link is hidden in a now-resolved thread but https://mypy.readthedocs.io/en/stable/generics.html#generic-methods-and-generic-self shows more or less exactly this, albeit with a warning that this is 'experimental' meaning that mypy might say wrong things.

[danieleades]

Pretty annoying to have to implement derived methods just for the sake of the typing, though?

absolutely. I don't have any particular issue with this, just not something i've seen in the wild.
Python really needs something like Rust's Self type

[NiklasRosenstein]

@danieleades A Self type is coming in Python 3.11 (PEP 673)