update: pygit2.remote.RemoteCallbacks -> pygit2.RemoteCallbacks

[milahu]

make it work with latest pygit2

error was

ImportError: cannot import name 'RemoteCallbacks' from 'pygit2.remote'

env

$ python --version 
Python 3.9.6

$ python -c 'import pygit2; print(pygit2.__version__)'
1.6.1

todo: patch requirements.txt

... or is this intentional?

[taylorjacklespriggs]

#379 includes the requirements.txt patch