Adds lib.pyproj file to solution

[zooba]

This allows VS 2015 or VS 2017 with Python support installed to open all the standard library files.

[mention-bot]

@zooba, thanks for your PR! By analyzing the history of the files in this pull request, we identified @briancurtin, @amauryfa and @shibturn to be potential reviewers.

[zooba]

VS 2015 (and to a much lesser extent, VS 2017) will hit memory issues if you load the project, so removing it from the solution. It's still there for people to load if they want.

[zooba]

Also, we tested the version number changes in the solution file and they don't impact VS 2015.

[terryjreedy]

The commit message is a bit confusing as it says that lib.pyproj was added to and then deleted from 'solution' for a net null effect. Reading the commit, it seems that the actual effect is

• Add 1000+ lines to a new or existing PCbuild/lib.pyproj.
• Update Visual Studio version in PCbuild/pcbuild.sln.