Remove dulwich requirement

Author: adamtheturtle

dev-requirements.txt

@@ -11,7 +11,6 @@ check-manifest==0.40
 codecov==2.0.15 # Upload coverage data
 doc8==0.8.0
 dodgy==0.1.9  # Look for uploaded secrets
-dulwich==0.19.14
 flake8-commas==2.0.0  # Require silicon valley commas
 flake8-quotes==2.1.1  # Require single quotes
 flake8==3.7.9  # Lint