Closes #14158: We now track test_support.TESTFN cleanup, and test_mailbox uses shutil.rmtree for simpler code.

Patch by Ralf Schmitt.

I assume that most users looking at this page are looking for a library
to process XML, not a low-level parser.  First proposed in #11379.

source generates a nifty link to the Mercurial web viewer.

Misc/COPYRIGHT was renamed twelve years ago to LICENSE and later copied
to the main docs, to which there is already a link.

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.