Drop support for python 3.7

[consideRatio]

At this point Python 3.6 has reached end of life. If we wish to support it, we may need to not use modern versions of libraries etc as they may no longer support Python 3.6.

Due to that, given agreement, I suggest we drop support for Python 3.6 at this point if there is agreement about it.

[martindurant]

Dask no longer supports 3.6, so fully support dropping it here

[jacobtomlinson]

We only support 3.8 and above. I would suggest we should be consistent here too.

dask/community#213

[consideRatio]

In #501 I've now added a commit to drop Python 3.6 and require Python 3.7+.

To drop support for Python 3.7 without breaking our test suite etc, some additional work needs to be done.

Action points

• Drop support for Python 3.6 (maint/ci: remove support for Python 3.6 and test against multiple versions of Python and Golang #501)
• Drop support for Python 3.7