python: Inline expectation should have space after $

This was a regex-find-replace from `# \$(?! )` (using a negative lookahead) to `# $ `.

python/ql/test/library-tests/frameworks/flask/session_interface.py

@@ -1,5 +1,5 @@
-import flask 
+import flask
 
 class MySessionInterface(flask.sessions.SessionInterface):
     def open_session(self, app, request):
-        ensure_tainted(request) # $tainted
+        ensure_tainted(request) # $ tainted