bpo-40275: test.support.check_impl_detail() uses sys.implementation

[vstinner]

check_impl_detail() of test.support now uses sys.implementation.name,
instead of platform.python_implementation().lower(). This change
prepares test.support to import the platform module lazily.

https://bugs.python.org/issue40275

[vstinner]

I tested manually: PyPy3 provides sys.implementation:

$ pypy3 -c 'import sys; print(sys.implementation.name)'
pypy
$ python3 -c 'import sys; print(sys.implementation.name)'
cpython

$ pypy3 -V
Python 3.6.9 (78d4c48fa091, Apr 30 2020, 07:55:31)
[PyPy 7.3.1 with GCC 10.0.1 20200328 (Red Hat 10.0.1-0.11)]
$ python3 -VV
Python 3.8.2 (default, Feb 28 2020, 00:00:00) 
[GCC 10.0.1 20200216 (Red Hat 10.0.1-0.8)]