[BEAM-547] [BEAM-1730] Cleanup versions.py and root init has version access

[sb2nov]

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R: @aaltay PTAL

[aaltay]

Do we need version.py after this change?
And can we remove https://github.com/apache/beam/blob/master/sdks/python/MANIFEST.in#L18

[coveralls]

Coverage decreased (-0.01%) to 70.183% when pulling f160325 on sb2nov:BEAM_547-simplify-version-python into 477e789 on apache:master.

[asfbot]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/beam_PreCommit_Java_MavenInstall/8474/
--none--

[sb2nov]

We still need versions.py as setup.py calls that script. I'll remove the manifest line

[sb2nov]

R: @aaltay PTAL

[coveralls]

Coverage decreased (-0.003%) to 70.18% when pulling 9a021b6 on sb2nov:BEAM_547-simplify-version-python into 004d307 on apache:master.

[asfbot]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/beam_PreCommit_Java_MavenInstall/8478/
--none--

[aaltay]

LGTM. Thank you.