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

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Also fixes an outdated note about _lzma in PCbuild/readme.txt

PCbuild/get_external.py

@@ -0,0 +1,60 @@
+#!/usr/bin/env python3
+
+import argparse
+import os
+import pathlib
+import zipfile
+from urllib.request import urlretrieve
+
+
+def fetch_zip(commit_hash, zip_dir, *, org='python', binary=False, verbose):
+    repo = f'cpython-{"bin" if binary else "source"}-deps'
+    url = f'https://github.com/{org}/{repo}/archive/{commit_hash}.zip'
+    reporthook = None
+    if verbose:
+        reporthook = print
+    zip_dir.mkdir(parents=True, exist_ok=True)
+    filename, headers = urlretrieve(
+        url,
+        zip_dir / f'{commit_hash}.zip',
+        reporthook=reporthook,
+    )
+    return filename
+
+
+def extract_zip(externals_dir, zip_path):
+    with zipfile.ZipFile(os.fspath(zip_path)) as zf:
+        zf.extractall(os.fspath(externals_dir))
+        return externals_dir / zf.namelist()[0].split('/')[0]
+
+
+def parse_args():
+    p = argparse.ArgumentParser()
+    p.add_argument('-v', '--verbose', action='store_true')
+    p.add_argument('-b', '--binary', action='store_true',
+                   help='Is the dependency in the binary repo?')
+    p.add_argument('-O', '--organization',
+                   help='Organization owning the deps repos', default='python')
+    p.add_argument('-e', '--externals-dir', type=pathlib.Path,
+                   help='Directory in which to store dependencies',
+                   default=pathlib.Path(__file__).parent.parent / 'externals')
+    p.add_argument('tag',
+                   help='tag of the dependency')
+    return p.parse_args()
+
+
+def main():
+    args = parse_args()
+    zip_path = fetch_zip(
+        args.tag,
+        args.externals_dir / 'zips',
+        org=args.organization,
+        binary=args.binary,
+        verbose=args.verbose,
+    )
+    final_name = args.externals_dir / args.tag
+    extract_zip(args.externals_dir, zip_path).replace(final_name)
+
+
+if __name__ == '__main__':
+    main()