mypy: Use the new --fast-parser option.

This makes mypy about 15% faster running on the Zulip codebase (from
7s=>6s on my laptop), which seems worth it for losing a couple files.

This option requires a new dependency, which we add to the
mypy-specific requirements.txt file.

tools/py3_test_reqs.txt

@@ -1,2 +1,3 @@
 six==1.10.0
 git+https://github.com/python/mypy.git@9bb5f2e0ebc90aac5d948d1a7ebbb1f5003dca8e
+typed-ast==0.5.1

tools/run-mypy

@@ -21,7 +21,9 @@ tools/deprecated/
 zproject/
 zerver/lib/actions.py
 zerver/lib/bugdown/fenced_code.py
+zerver/lib/rate_limiter.py
 zerver/lib/statistics.py
+zerver/middleware.py
 zerver/migrations/
 zerver/tests/
 zerver/views/webhooks/jira.py
@@ -53,7 +55,7 @@ else:
     mypy_command = "mypy"
 
 if python_files:
-    rc = subprocess.call([mypy_command, "--silent-imports", "--py2", "--check-untyped-defs"] + python_files)
+    rc = subprocess.call([mypy_command, "--fast-parser", "--silent-imports", "--py2", "--check-untyped-defs"] + python_files)
     if not rc:
         print("The mypy static type checker for python detected no errors!")
     else: