Issue #13237: Forward port subprocess module updates and explicitly document UTF-8 encoding assumption when universal_newlines=True

Issue #13340.

Issue #10570

error handler in interactive mode (when calling into PyOS_Readline()).

Closes #13340.

The find utility of OpenBSD doesn't support the "-exec cmd {} +"
action. Change it to use "-depth -exec cmd {} ;" instead.

Also, remove __pycache__ before *.py[co], as most bytecode files are
in __pycache__ directories. Only those generated by Python 2 under
Doc/tools are not.

Closes #13326.

Closes #13254.

variable in the default value of a keyword-only argument:
(lambda *, arg=GLOBAL_NAME: None)

Thanks to Robert Xiao.

Unicode string.

This is an incompatible change, but putp() is used to emit terminfo commands,
which are bytes strings, not Unicode strings.

New wording is taken straight from the PEP, so this time should be good
:)

Unicode string.

This is an incompatible change, but the previous behaviour was completly wrong.

- 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 smells like a disabled test
  method instead of an helper method
- Fix some idioms (assertIn, addCleanup)

#670664: Fix HTMLParser to correctly handle the content of ``<script>...</script>`` and ``<style>...</style>``.