tests: simplify Windows and PYTHON3 conditionals

PYTHON3 is always True. So this flow can be reduced.

Differential Revision: https://phab.mercurial-scm.org/D12243

tests/run-tests.py

@@ -3562,10 +3562,8 b' class TestRunner(object):'
     def _usecorrectpython(self):
         """Configure the environment to use the appropriate Python in tests."""
         # Tests must use the same interpreter as us or bad things will happen.
-        if WINDOWS and PYTHON3:
+        if WINDOWS:
             pyexe_names = [b'python', b'python3', b'python.exe']
-        elif WINDOWS:
-            pyexe_names = [b'python', b'python.exe']
         else:
             pyexe_names = [b'python', b'python3']