Use flake8 instead of pyflakes and pep8

It runs both of them.

About the `devscripts/buildserver.py` errors:
```
./devscripts/buildserver.py:123:9: F821 undefined name 'TODO'
./devscripts/buildserver.py:127:34: F821 undefined name 'service_name'
./devscripts/buildserver.py:132:17: F821 undefined name 'SERVICE_STATUS'
./devscripts/buildserver.py:153:9: F821 undefined name 'TODO'
./devscripts/buildserver.py:234:1: E265 block comment should start with '# '
./devscripts/buildserver.py:254:19: F821 undefined name '_winreg'
./devscripts/buildserver.py:254:35: F821 undefined name '_winreg'
./devscripts/buildserver.py:256:38: F821 undefined name '_winreg'
./devscripts/buildserver.py:258:17: F821 undefined name '_winreg'
./devscripts/buildserver.py:278:25: F821 undefined name 'tempfile'
./devscripts/buildserver.py:287:13: F821 undefined name 'subprocess'
./devscripts/buildserver.py:288:13: F821 undefined name 'subprocess'
./devscripts/buildserver.py:289:16: F821 undefined name 'subprocess'
./devscripts/buildserver.py:308:13: F821 undefined name 'subprocess'
./devscripts/buildserver.py:310:16: F821 undefined name 'subprocess'
./devscripts/buildserver.py:339:13: F821 undefined name 'shutil'
./devscripts/buildserver.py:376:16: F821 undefined name 'urlparse'
./devscripts/buildserver.py:377:61: F821 undefined name 'urlparse'
./devscripts/buildserver.py:403:1: E265 block comment should start with '# '
```

The `TODO` should probably be a comment, other seem to be non imported modules. I could fix them, but I don't know how to test it.