Gevent-based applications need to be able to import threading, socket,
ssl, etc... modules early to be able to monkey-patch them to use
lightweight greenlets instead of OS threads. This patching has to be
done as early as possible in the lifetime of the process

http://www.gevent.org/intro.html#monkey-patching

However starting from caae48b (use import hooks to patch
distutils/setuptools (pypa#1688)) threading became always preimported which,
for example, rendered gpython[1] unusable:

	(py38.virtualenv) kirr@deco:~/tmp/trashme$ gpython
	Traceback (most recent call last):
	  File "/home/kirr/tmp/trashme/py38.virtualenv/bin/gpython", line 8, in <module>
	    sys.exit(main())
	  File "/home/kirr/tmp/trashme/py38.virtualenv/lib/python3.8/site-packages/gpython/__init__.py", line 151, in main
	    raise RuntimeError('gpython: internal error: the following modules are pre-imported, but must be not:'
	RuntimeError: gpython: internal error: the following modules are pre-imported, but must be not:

	        ['threading']

	sys.modules:

	        ['__future__', '__main__', '_abc', '_bootlocale', '_codecs', '_collections', '_collections_abc', '_frozen_importlib', '_frozen_importlib_external', '_functools', '_heapq', '_imp', '_io', '_locale', '_operator', '_signal', '_sitebuiltins', '_sre', '_stat', '_thread', '_virtualenv', '_warnings', '_weakref', '_weakrefset', 'abc', 'builtins', 'codecs', 'collections', 'contextlib', 'copyreg', 'encodings', 'encodings.aliases', 'encodings.latin_1', 'encodings.utf_8', 'enum', 'functools', 'genericpath', 'gpython', 'heapq', 'importlib', 'importlib._bootstrap', 'importlib._bootstrap_external', 'importlib.abc', 'importlib.machinery', 'importlib.util', 'io', 'itertools', 'keyword', 'marshal', 'operator', 'os', 'os.path', 'posix', 'posixpath', 're', 'reprlib', 'site', 'sitecustomize', 'sre_compile', 'sre_constants', 'sre_parse', 'stat', 'sys', 'threading', 'time', 'types', 'warnings', 'zipimport', 'zope']

Fix it by importing threading lazily.

Fixes: pypa#1895

[1] https://pypi.org/project/pygolang/#gpython

AttributeError("'PosixPath' object has no attribute 'rfind'",)

Address review feedback:

- wrap comments at 120 characters;
- use "imported" instead of "preimport" in tests and leverage python
  syntax sugar to raise signal/noise;
- inline links in changelog

Try to fix test failures:

- try to disable coverage for spawned subrocess.

  However in the presence of coverage-enable-subprocess coverge is still
  unconditioanlly imported, and even plain `import coverage` leads to
  `import threading`. Probably need to fix coverage as well.

Disable coverage for the test as suggested by @gaborbernat.

Xfail on pypy3/darwin. See reason link for details.