Prove that `PluginManger.register()` ignores non-routines
Prove that `PluginManger.register()` ignores non-routines

This proves that #91 is a problem in `pytest` and not `pluggy`.
Thanks to @RonnyPfannschmidt for the fix about a year ago ;)

hpk42/pluggy,pytest-dev/pluggy,RonnyPfannschmidt/pluggy,tgoodlet/pluggy,RonnyPfannschmidt/pluggy,pytest-dev/pluggy,nicoddemus/pluggy