github/workflows: Improve MSYS2-based CI builds.

Install the mingw variant of Python since it behaves more like a 'real'
Windows CPython than the msys2 variant: os.name == 'nt', not 'posix'.  Note
that os.sep is still '/' though so we don't actually need to skip the
import_file test.  This way one single Python version can be used both for
running run-tests.py and getting the expected test output.

Signed-off-by: stijn <stijn@ignitron.net>

tests/run-tests.py

@@ -3,6 +3,7 @@
 import os
 import subprocess
 import sys
+import sysconfig
 import platform
 import argparse
 import inspect
@@ -583,10 +584,6 @@ def run_tests(pyb, tests, args, result_dir, num_threads=1):
             # fails with stack overflow on Debug builds
             skip_tests.add("misc/sys_settrace_features.py")
 
-            if os.getenv("MSYSTEM") is not None:
-                # fails due to wrong path separator
-                skip_tests.add("import/import_file.py")
-
     if upy_float_precision == 0:
         skip_tests.add("extmod/uctypes_le_float.py")
         skip_tests.add("extmod/uctypes_native_float.py")
@@ -711,7 +708,9 @@ def run_tests(pyb, tests, args, result_dir, num_threads=1):
 
     # Some tests use unsupported features on Windows
     if os.name == "nt":
-        skip_tests.add("import/import_file.py")  # works but CPython prints forward slashes
+        if not sysconfig.get_platform().startswith("mingw"):
+            # Works but CPython uses '\' path separator
+            skip_tests.add("import/import_file.py")
 
     # Some tests are known to fail with native emitter
     # Remove them from the below when they work