bpo-30450: Pull Windows dependencies from GitHub rather than svn (GH-1783) (GH-2237)

bpo-30450: Pull Windows dependencies from GitHub rather than svn (GH-1783) (GH-2237)

The Windows build now depends on Python 3.6 to fetch externals, but it will be downloaded via NuGet (which is downloaded via PowerShell) if it is not available via `py -3.6`.  This means the only thing that must be installed on a modern Windows box to do a full build of CPython with all extensions is Visual Studio.

Also fixes an outdated note about _lzma in PCbuild/readme.txt

(cherry-picked from commit 51599e2bdd10ab77212a7cbb41a13ea70ee13da8)