Raise minimum Python version

We are currently setting Python 3.6 as the minimum, which is already out of support.

We should raise at least to Python 3.7.

Best would be to raise the version of any other project using this lib to set Python 3.7 as the minimum as well, like tx-mining-service.

When we do this, we should also fix this:
- https://github.com/HathorNetwork/python-hathorlib/pull/10#discussion_r855953393

But there may be some advantages of going directly to 3.8, as:
- https://github.com/HathorNetwork/python-hathorlib/pull/10#discussion_r854342398