Issue #23684: Clarify the return value of the scheme attribute of ParseResult and SplitResult objects.

Patch by Martin Panter.

Patch by Michael Ensslin.

Based on patch by Martin Panter.

I would just outright delete them, but the readme in PCbuild recommended
their use, so I figure it would be nice to leave them there for a while.

It now recommends PCbuild/get_externals.bat instead of the scripts in
Tools/buildbot.

Related Issues:

#issue21528
#issue24453

and set comparisions.  Patch by Gareth Rees.

* Don't use the timestamp of an existing file anymore, only use fixed
  timestamp
* Enhance the code checking the resolution of the filesystem timestamps.
* Check timestamps with a resolution of 1 microsecond instead of 1 millisecond
* When os.utime() uses the current system clock, tolerate a delta of 20 ms.
  Before some os.utime() tolerated a different of 10 seconds.
* Merge duplicated tests and simplify the code

This is mostly a backport of issue #21907, but also includes a few
extras necessary to make the bulidbot scripts as thin as possible.

  string.Template instances.

Initial patch by Brandon Milam.

"cum" is not a valid argument.

Patch by Brandon Milam.

Ensure that the uninstall helper for Windows passes the proper
flags to pip to prevent it from checking PyPI if the pip that
we're currently attempting to uninstall is the latest verison.