[python] Add macOS big sur compatibility

[atomiix]

In macOS big sur, sysconfig.get_config_var('MACOSX_DEPLOYMENT_TARGET') is returning 11 as an integer which is incompatible with pkg_resources.parse_version().

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[atomiix]

@googlebot I signed it!

[lumaxis]

Running into this too and the fix looks good to me 👍🏼
@atomiix Looks like this PR needs the python label to make the bot happy.

[atomiix]

@lumaxis I'd like to but it seems that I have no right to add labels 😕

[acozzette]

Thanks, @atomiix