run-tests: Elaborate python3 command override capabilities.

tests/run-tests

@@ -5,8 +5,11 @@ import subprocess
 import sys
 from glob import glob
 
+# Tests require at least CPython 3.3. If your default python3 executable
+# is of lower version, you can point MICROPY_CPYTHON3 environment var
+# to the correct executable.
 if os.name == 'nt':
-    CPYTHON3 = 'python3.exe'
+    CPYTHON3 = os.getenv('MICROPY_CPYTHON3', 'python3.exe')
     MP_PY = '../windows/micropython.exe'
 else:
     CPYTHON3 = os.getenv('MICROPY_CPYTHON3', 'python3')