Switch //tools/md_browser to use cipd.

There are two standalone markdown-parsing tools in the repo,
one in //tools/md_browser and the other in //net/tools/net_docs.
Each uses a checked-in third-party copy of the Python markdown
library (two different versions).

Best practice these days for handling third-party packages of Python
code is to use CIPD if you can; this tightens up the
provenance better, hides the dependencies from the calling
code, and allows the calling code to just import the library
directly without doing any sys.path hacking.

We can't use CIPD to remove both uses of the library, as one
of the two uses happens during the build, and we're not allowed
to use vpython as a build action.

This change is motivated by crbug.com/14038, where we upgraded
//third_party/Python-Markdown from v3.3.4 to v3.4.1, but we
missed that the new version added a dependency on
`importlib-metadata`. Using CIPD packages would've caught that.

This CL switches md_browser to use vpython and CIPD, and
removes one of the two checked-in copies.

Bug: 140398
Change-Id: I583603e4c1318019b1b15391f52f989f4f8545f5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4283145
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/main@{#1109359}
NOKEYCHECK=True
GitOrigin-RevId: 2983b3e42e6356ef5c9af903bf2c4a484aca7de6
diff --git a/md_browser.py b/md_browser.py
index a7b3058..a0dfe46 100755
--- a/md_browser.py
+++ b/md_browser.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python3
+#!/usr/bin/env vpython3
 # Copyright 2015 The Chromium Authors
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
@@ -20,12 +20,10 @@
 import webbrowser
 from xml.etree import ElementTree
 
+import markdown
 
 THIS_DIR = os.path.realpath(os.path.dirname(__file__))
 SRC_DIR = os.path.dirname(os.path.dirname(THIS_DIR))
-sys.path.insert(0, os.path.join(SRC_DIR, 'third_party', 'Python-Markdown'))
-import markdown
-
 
 def main(argv):
   parser = argparse.ArgumentParser(prog='md_browser')