Only complain if the config target is >= 10.3 and the current target is
< 10.3. The check was originally added to ensure that incompatible
LDSHARED flags are not used, because -undefined dynamic_lookup is
used when building for 10.3 and later, and is not supported on older OS
versions. Apart from that, there should be no problem in general
with using an older target.

Authored-by: Joshua Root <jmr@macports.org>

* bpo-42504: Ensure that get_config_var('MACOSX_DEPLOYMENT_TARGET') is a string

macOS releases numbering has changed as of macOS 11 Big Sur.  Previously, major releases were of the form 10.x, 10.x+1, 10.x+2, etc; as of Big Sur, they are now x, x+1, etc, so, for example, 10.15, 10.15.1, ..., 10.15.7, 11, 11.0.1, 11.1, ..., 12, 12.1, etc. Allow Python to build with single-digit deployment target values. Patch provided by FX Coudert.

Automerge-Triggered-By: @jaraco

Reimplement distutils.spawn.spawn() function with the subprocess
module.

setup.py now uses a basic implementation of the subprocess module if
the subprocess module is not available: before required C extension
modules are built.

shutil.which() and distutils.spawn.find_executable() now use
os.confstr("CS_PATH") if available instead of os.defpath, if the PATH
environment variable is not set.

Don't use os.confstr("CS_PATH") nor os.defpath if the PATH
environment variable is set to an empty string to mimick Unix 'which'
command behavior.

Changes:

* find_executable() now starts by checking for the executable in the
  current working directly case. Add an explicit
  "if not path: return None".
* Add tests for PATH='' (empty string), PATH=':' and for PATHEXT.

distutils.spawn.find_executable() now falls back on os.defpath if the
PATH environment variable is not set.