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.

Patch by Geert Jansen.

Some time ago we changed the docs to consistently use the term 'bytes-like
object' in all the contexts where bytes, bytearray, memoryview, etc are used.
This patch (by Ezio Melotti) completes that work by changing the error
messages that previously reported that certain types did "not support the
buffer interface" to instead say that a bytes-like object is required.  (The
glossary entry for bytes-like object references the discussion of the buffer
protocol in the docs.)

* 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.

* 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.