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 51599e2bdd, parts of 40a23e8899, parts of 68d663cf85, d5cd21d75a, 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
Zachary Ware