Backfill .. versionadded and .. versionchanged in the API reference

[pradyunsg]

This would make it easier for users to spot when a function is added to the API (eg: I had to spend an extra bit of effort and find the git blame for when we added platform_tags, instead of being able to see that in the docs itself).

See https://packaging.pypa.io/en/latest/changelog.html for the changelog.

• 21.3 (Add versionchanged for 21.3 #599)
• 21.2
• 21.1
• 21.0
• 20.9
• 20.8
• 20.7
• 20.6
• 20.5
• 20.4
• 20.3
• 20.2
• 20.1
• 20.0
• 19.2
• 18.0
• 17.1
• 17.0
• 16.8
• 16.7
• 16.6
• 16.5
• 16.4
• 16.3
• 16.2
• 16.1
• 16.0
• 15.3
• 15.2
• 15.1
• 15.0
• 14.5
• 14.4
• 14.3
• 14.2
• 14.1
• 14.0

[pradyunsg]

Hmm... we seem to be missing the platform_tags change from the changelog even. >.<

This might end up requiring changelog updates as well.