…) (python#159264)

* test_cext now uses a single virtual environment for all tests.
  It makes the test way faster since creating a virtual environment
  is quite slow.
* Add Py_TARGET_ABI3T test on C++.
* Move macro tests from module exec function to test_macros().
* Call Py_BEGIN_CRITICAL_SECTION() on a fresh dictionary object,
  rather than on the module.
* Rename some environment variables to always use "CPYTHON_TEST_"
  prefix.
* Replace support.MS_WINDOWS with MSVC to be more explicit.
* Move macro tests to test_macros().
* On macOS, use C++11 by default.

(cherry picked from commit b71989d)
(cherry picked from commit 364db5d)