py3: replace os.environ with encoding.environ (part 2 of 5)

py3: replace os.environ with encoding.environ (part 1 of 5) os.environ is a dictionary which has string elements on Python 3. We have encoding.environ which take care of all these things. This is the first patch of 5 patch series which tend to replace the occurences of os.environ with encoding.environ as using os.environ will result in unusual behaviour.

tests: exclude bundled pywatchman from check-code test pywatchman is imported from upstream and therefore fails to pass linting. We have added 'no-check-code' manually to every file in the past. This is cumbersome and modifies upstream sources.