Fix new issues with unit tests

- Create custom test suite based on platform
- Use the custom suite for all tests
- Update tox.ini to install pywin32

tests/__init__.py

@@ -0,0 +1,18 @@
+import sys
+import unittest
+
+
+def TestSuite():
+    suite = unittest.TestSuite()
+    loader = unittest.TestLoader()
+    if sys.platform == "win32":
+        from . import test_plat_win
+
+        suite.addTests(loader.loadTestsFromModule(test_plat_win))
+    else:
+        from . import test_script_main
+        from . import test_plat_other
+
+        suite.addTests(loader.loadTestsFromModule(test_script_main))
+        suite.addTests(loader.loadTestsFromModule(test_plat_other))
+    return suite