Initial patch by Johannes Vogel. Issue #13491.

Issue #14159: Fix the len() of weak sets to return a better approximation when some objects are dead or dying.
Moreover, the implementation is now O(1) rather than O(n).
Thanks to Yury Selivanov for reporting.

#10713: Improve documentation for \b and \B and add a few tests.  Initial patch and tests by Martin Pool.

Issue #13521: dict.setdefault() now does only one lookup for the given key, making it "atomic" for many purposes.
Patch by Filip Gruszczyński.

- Actually check the contents of the file created by bdist_dumb.
- Don’t use “RECORD” as filename for non-PEP 376 record file
- Don’t start method name with “_test”, it looks like a disabled test
  method instead of an helper method
- Fix some idioms (assertIn, addCleanup)

Requested by Florent Xicluna with the rationale that they make the docs
look unfinished.  When I get to replace the XXX notes with the real info
for packaging, I’ll backport it.

Also removed a few XXX notes that were not visible in the HTML but could
waste contributors’ time by suggesting improvements that are never going
to happen for distutils.

Bug reported by Hartmut Goebel and patch contributed by Carl Robben.
Untested backport of the fix committed and tested for 3.2.

- Remove duplicate list of links to the other modules from each
  module’s doc (people can already go up to library/archiving and
  there they can see the list).

- Link to shutil high-level operations.

Reviewed by Nadeem Vawda in #6715.

When people find bugs in scripts such as reindent.py, msgfmt.py or
pygettext.py, we have to try to reproduce the bug manually, apply a fix
and test manually again.  The alternative is to only read the code and
trust that it works.  This test file is a way to stop that
unsatisfactory state of things and write proper unit tests instead.

These regex changes fix a number of issues for distutils on Windows:
- #6884: impossible to include a file starting with 'build'
- #9691 and #14004: sdist includes too many files
- #13193: test_filelist failures

This commit replaces the incorrect changes done in 557a973709de,
c566a3447ba1 and 3925081a7ca0 to fix #13193; we were too eager to fix
the test failures and I did not study the code enough before
greenlighting patches.  This time we have unit tests from the problems
reported by users to be sure we have the right fix.

Thanks to Nadeem Vawda for his help.