Testing depends on alpha sumtypes #1797

Since commit 7ff3cee GitPython's tests started to depend on sumtypes which does not have any stable release available at PyPI and seems to be dead for more than two years now (see https://github.com/radix/sumtypes/commits/master/).

This causes inconvenience for downstream packagers where tests are run to make sure the constructed package is working properly. Could you please make the sumtypes test dependency optional (especially on non-Windows OSes)? Or switch off of it completely?

Thank you.