Fix tests on Windows

Author: AWhetter

mypy/test/testcmdline.py

@@ -59,7 +59,7 @@ def test_python_cmdline(testcase: DataDrivenTestCase, step: int) -> None:
     extra_path = os.path.join(os.path.abspath(test_temp_dir), 'pypath')
     env['PYTHONPATH'] = PREFIX
     if os.path.isdir(extra_path):
-        env['PYTHONPATH'] += ':' + extra_path
+        env['PYTHONPATH'] += os.pathsep + extra_path
     process = subprocess.Popen(fixed + args,
                                stdout=subprocess.PIPE,
                                stderr=subprocess.PIPE,