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.

Patch by Christian Heimes.