github_activity plugin: CRITICAL: TypeError:

[visibilityspots]

Using pelican 3.6.3 with python 3.5.1 the make html fails when enabling the github_activity plugin:

CRITICAL: TypeError: a bytes-like object is required, not 'str'
Makefile:49: recipe for target 'html' failed
make: *** [html] Error 1

[klreeher]

I've got Pelican 3.7.1 and Python 3.6.0 and I'm getting the very similar error when I attempt to enable Filetime_from_git.

ERROR: Could not process .\article.md | TypeError: a bytes-like object is required, not 'str'

[justinmayer]

@visibilityspots / @klreeher: Would you consider submitting a PR to address the issue(s) discussed here?