Use bash arrays with one entry per line for easier handling.

archlinux/devtools!109 (comment 74475)

Switch to signed git tags, as upstream has broken signatures for pypi sdist tarballs.
Switch to PEP517.
Remove unneeded quotes and curly braces.

Fixes https://bugs.archlinux.org/task/69717

Upstream switched to new key-id 27C50E7F590947D7273A741E85194C08421980C9
after breaking their hardware key which carried the previous:
https://github.com/gitpython-developers/gitdb/issues/55

Upstream is still using a new, unknown, unavailable GPG key for signing releases.
Due to a messed up source array this has not been noticed, but is now reverted.

Switching to old (correct) pypi project gitdb, as apparently upstream is using this again.
Updating took quite long this time due to trust issues:
https://github.com/gitpython-developers/gitdb/issues/55

Upgrading to 2.0.6. Enabling tests by creating a dummy git repository in accordance with upstream recommendation (one performance test still remains disabled).

Dropping python2 support. Adding (but disabling) tests and adding upstream ticket for them failing. Updating maintainer. Minor cleanups.