Fixes several small bugs:

• issues with np.complex for numpy versions > 1.20 (#236, #220). Thanks @SaftMacki for the fix and @oslopanda and @pililac for pointing out the issues! ☄️
• max_M could be off by one in linKK tests. Thanks @markbouman! 🧪
• frequency and impedance inputs can now be np.array in addition to list. Thanks @edmundsj for the suggestion!

Also added publishing to PyPI via GitHub Actions (so hopefully these releases come faster and can be made by more folks) and removed support for EOL Python v3.6