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.

Lib/test/test_imp.py

@@ -210,6 +210,10 @@ class PEP3147Tests(unittest.TestCase):
         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

Python/import.c

@@ -943,12 +943,12 @@ make_compiled_pathname(Py_UNICODE *pathname, int debug)
     Py_UNICODE_strcat(buf, CACHEDIR_UNICODE);
     i += Py_UNICODE_strlen(CACHEDIR_UNICODE) - 1;
     buf[i++] = sep;
-    buf[i++] = '\0';
+    buf[i] = '\0';
     /* Add the base filename, but remove the .py or .pyw extension, since
        the tag name must go before the extension.
     */
     Py_UNICODE_strcat(buf, pathname + save);
-    pos = Py_UNICODE_strrchr(buf, '.');
+    pos = Py_UNICODE_strrchr(buf + i, '.');
     if (pos != NULL)
         *++pos = '\0';
     Py_UNICODE_strcat(buf, PYC_TAG_UNICODE);