servo: Merge #6279 - Add flake8 to the tidy process for Python files (from frewsxcv:flake8); r=jdm

Fixes #6236

Also included in this commit are the changes need to make flake8 pass
for the existing python file

Source-Repo: https://github.com/servo/servo
Source-Revision: ccfe29d8f284dedc0101045d574a98fb1f69aa62

servo/python/mach_bootstrap.py

@@ -98,7 +98,7 @@ def bootstrap(topdir):
 
     for category, meta in CATEGORIES.items():
         mach.define_category(category, meta['short'], meta['long'],
-            meta['priority'])
+                             meta['priority'])
 
     for path in MACH_MODULES:
         mach.load_commands_from_file(os.path.join(topdir, path))