Egg race condition

Switch over to using six.PY{2,3} when possible

* Add a space after header, so reST rendering isn't mangled.

* Add newline in template, too.

This fixes a race condition in _bypass_ensure_directory where
two threads or processes may erroneously fail because they are
both creating the same directory. A more robust implementation
of this may involve exposing the un-wrapped os.makedirs.

Originally reported with proposed patch by @JonKohler in
github PR #1412. This patch came out of discussions on that
thread.

This mocks out the isdir call so that the directory is created
immediately after you determine whether or not it exists, thus
simulating a race condition between two threads or processes
creating the same directory.

fix #97 PEP420: find_packages()

Fix "Microsoft Visual C++ Build Tools" link

Deprecate upload and register commands

This moves the documentation out of "Basic Usage" and expands on some of
the caveats of this approach.

This fixes GH #97 by introducing an alternate version of find_packages
that works with PEP 420 namespace packages.

docs: fix developer guide

fix PEP 518 configuration

Add `build-system.requires` key to `pyproject.toml`:
- this is necessary with pip 10.0.1, as otherwise the defaults will be
  to require both `setuptools` and `wheel` (and we only need the later)
- a `pyproject.toml` with no `build-system.requires` key will be invalid
  and rejected by a future version of pip
  (see https://github.com/pypa/pip/issues/5416#issuecomment-399638608)

Drop `Source Code` section, as it contains an outdated guideline
with respect to documenting changes and its contents are already
covered by other sections.

Avoid resource warning from dev scripts installed editable

Avoid ` ResourceWarning: unclosed file <_io.TextIOWrapper name='whatever' mode='r' encoding='UTF-8'>` on dev scripts installed editable

Fix Town Crier Link