Combine notes from RCs into X.Y.Z.rst

[berkerpeksag]

Currently, 2.7.15.rst only shows changes between 2.7.15rc1 and 2.7.15 final, not between 2.7.14 final and 2.7.15 final. I think this creates a bit confusion and breaks the old workflow. We can't now just link to https://github.com/python/cpython/blob/2.7/Misc/NEWS.d/2.7.15.rst at https://www.python.org/downloads/release/python-2715/ since it doesn't combine changes from https://github.com/python/cpython/blob/2.7/Misc/NEWS.d/2.7.15rc1.rst and https://github.com/python/cpython/blob/2.7/Misc/NEWS.d/2.7.15.rst.

See python/pythondotorg#1252 for more details.

[Mariatta]

GitMate.io thinks possibly related issues are python/core-workflow#166 (blurb and garglemox ?), python/miss-islington#38 (Remove [X.Y] from the backport commit message), python/core-workflow#210 (blurb loses information when merge), python/devguide#290 (Update buildslave.rst page for Buildbot 0.9.x), and python/core-workflow#247 (blurb can break a line after a hyphen).

[larryhastings]

Please see python/pythondotorg#1252 (comment) for my comments.