Ignore E203 (Whitespace before ':')

As explained in this issue[1], E203[2] is not PEP 8 compliant so it
should be ignored when running flake8.

Black is now being used in the codebase, and it takes care of the
formatting.

[1] psf/black#315
[2] https://www.flake8rules.com/rules/E203.html

checkbox-ng/plainbox/provider_manager.py

@@ -1615,7 +1615,7 @@ def create_flake8_test(pyfile):
     """Creates the target for the monkey patched methods in Flake8Tests"""
 
     def run_flake8(self):
-        result = subprocess.run(["flake8", pyfile])
+        result = subprocess.run(["flake8", "--extend-ignore=E203", pyfile])
         self.assertEqual(result.returncode, 0)
 
     return run_flake8