Fix imp.cache_from_source() if the directory name contains a dot

If the directory name contains a dot but not the filename, don't strip at the
dot.
diff --git a/Lib/test/test_imp.py b/Lib/test/test_imp.py
index d745ae9..83e17d3 100644
--- a/Lib/test/test_imp.py
+++ b/Lib/test/test_imp.py
@@ -210,6 +210,10 @@
         self.assertEqual(
             imp.cache_from_source('/foo/bar/baz/qux.py', True),
             '/foo/bar/baz/__pycache__/qux.{}.pyc'.format(self.tag))
+        # Directory with a dot, filename without dot
+        self.assertEqual(
+            imp.cache_from_source('/foo.bar/file', True),
+            '/foo.bar/__pycache__/file{}.pyc'.format(self.tag))
 
     def test_cache_from_source_optimized(self):
         # Given the path to a .py file, return the path to its PEP 3147