Use sys.implementation.name if available

[pradyunsg]

From https://www.python.org/dev/peps/pep-0425/#python-tag

Other Python implementations should use sys.implementation.name.

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We should probably use sys.implementation.name in packaging.tags, instead of platform.python_implementation().lower(), if it exists.

packaging/packaging/tags.py

Line 341 in d09b0b5

name = platform.python_implementation().lower()

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Relevant discussion about platform.python_implementation() vs sys.implementation: https://www.python.org/dev/peps/pep-0421/#platform-python-implementation