TODOs (mostly to do after add tests), and make a few other changes.

Remove the advice to never use relative imports; it is a leftover from 2.x implicit relative imports.
Remove the advice to locally import modules in __init__, it is a strange practice.
Remove the advice to use "from ... import *" with some modules.

Clean up the docs of PyObject_IsSubclass and PyObject_IsInstance, and mention that they call the PEP 3119 methods.

* Don't call the garbage collector with an exception set: it causes an
  assertion to fail in debug mode.
* Enhance also error handling if allocating an array for the executable list
  failed.
* Add an unit test for 4 different errors in the _posixsubprocess module.

Original patch by Erik Johansson, slightly updated by Meador Inge.

Original patch by Peter Saveliev.

Issue #21905: Avoid RuntimeError in pickle.whichmodule() when sys.modules is mutated while iterating.

Patch by Olivier Grisel.

"xmlcharrefreplace", and "surrogatepass" error handlers.

(including empty directories) in ZIP file.

Patch by Éric Araujo with help from Lars Gustäbel.

Patch by Christian Heimes and Alex Gaynor.

Reported by Scott Hinton on docs@.

The test needs to be skipped if unicodedata is either part of the
main binary (a repackaging of cpython on Windows?) or has python
source (pypy?).  PEP 3147 makes __file__ point to the .py source,
so we need to change the extension check from looking for the
old .pyc/.pyo to just looking for .py.

Note that this skip should never trigger on CPython itself, so
one could argue it should be dropped instead.  But since it exists,
why risk breaking someone else's python.

Python3 does not have the bug covered by the issue.

derived from sr_rs@latin.