Use sys.version_info instead of sys.version in packaging.

Use sys.version_info instead of sys.version in packaging.

The contents of this attribute are an implementation detail, as
documented for #9442, so we should not parse it, to support non-CPython
VMs with distutils2 in the future.

Unfortunately, one use comes directly from PEP 345, so an edit will have
to be agreed before fixing the code (see comment in p7g.markers).

Other remaining uses are found in p7g.compiler and could be replaced by
the platform module (which also parses sys.version, but then it wouldn’t
be my fault :)