Use cache_tag in default build_platlib dir

[mgorny]

Use sys.implementation.cache_tag instead of Python version to create
the default directory for build_platlib. This guarantees that
the directories used by CPython and PyPy are distinct. Prior to
the change, both CPython and PyPy would use e.g. lib.linux-x86_64-3.9.
With the change, they are going to use lib.linux-x86_64-cpython39
and lib.linux-x86_64-pypy39 respectively.

[kmod]

Hello, I'd like to advocate for a different name for these directories. There is a bit of a convention of using sysconfig.get_config_var("VERSION") for this purpose instead of sys.implementation.cache_tag, so these new cache directories will be named differently than most of the ecosystem expects. For example

• within distutils
• how uwsgi does it

These directory names are used for significantly more than caching, so I believe VERSION is a bit more appropriate. See #107 for a bit of rationale and my proposed code change.