Workaround mypy un-ignoreable error in Py35

Author: lordmauve

src/tests/test_cases/fstrings.py renamed to src/tests/test_cases/fstrings.py36

@@ -36,6 +36,18 @@ def test_all_interpreters(test_case):
 )
 def test_fstrings():
     """Run the f-string test case under Python 3.6+ only."""
+    # When run under Python 3.5, mypy reports a parse error for the test file,
+    # because Python 3.5 doesn't support f-strings. It does not support
+    # ignoring parse errors.
+    #
+    # To work around this, we put our code in a file that mypy cannot see. This
+    # code reveals it to Python.
+    from . import test_cases
+    import importlib.machinery
+    test_cases_dir = test_cases.__path__[0]
+    loader = sys.path_importer_cache[test_cases_dir]
+    loader._loaders.append(('.py36', importlib.machinery.SourceFileLoader))
+
     run_case('fstrings')
 
 
@@ -46,10 +58,8 @@ def run_case(test_case):
                              locals=locals(),
                              fromlist=['expectation'],
                              level=1)
-    test_case_dir = os.path.normcase(os.path.dirname(__file__))
-    test_case_file = os.path.join(test_case_dir,
-                                  'test_cases',
-                                  test_case + '.py')
+
+    test_case_file = case_module.__file__
     results = list(check([test_case_file],
                          select=set(ErrorRegistry.get_error_codes()),
                          ignore_decorators=re.compile(