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

Review the project's Python-version configuration and the linked pull request discussions, then check how tx-mining-service sets its minimum version. Resolve whether the project should require Python 3.7 or 3.8, update the related project requirements, and address the linked discussion before considering the issue done.

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