Fix coveralls integration
Fix coveralls integration

See https://github.com/TheKevJames/coveralls-python/issues/252: the
coveralls API requires a service name, which used to be 'github', but a
new release of https://pypi.org/p/coveralls switched the default to
'github-actions'.

Nobody understands the exact difference between the two, but I have 19
repositories that now fail coveralls uploads using the 'github-actions'
service name, and I hope that switching to 'github' will fix them.

mgedmin/objgraph