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.

Cherry-picked from 51599e2b, parts of 40a23e88, parts of 68d663cf, d5cd21d7, and possibly others that I've missed.

Also:

* Rename db -> bsddb for disambiguity

* Update sqlite3 to 3.14.2.0 since it's the version we use on 3.x, and it's simpler to just use it than to also upload the old version to cpython-source-deps

* Add PCbuild/*.ilk to .gitignore