Use local tidy

#861 (comment)

"I think the most important concern is that it's possible to modify tidy.py
and see how those changes affect ./mach test-tidy with the fewest possible
intermediate steps." - jdm

This takes publishing complexity away from the contributor when testing
changes and makes it an infra problem instead, where it's much easier to
automate & saner to manage pypi credentials

python/requirements.txt

@@ -14,4 +14,4 @@ pyflakes == 0.8.0
 # For test-webidl
 ply == 3.8
 
-servo_tidy == 0.0.1
+-e python/tidy