Bump min version support

[blink1073]

Drop support for Python 3.7 and bump min version support.
Also update example notebook to use new comm package.

Fixes failing test:

>           assert normalized_expected_outputs == normalized_actual_outputs
E           assert [] == [{'output_type': 'stream', 'name': 'stderr', 'text': "<IPY-INPUT>:1: DeprecationWarning: The `ipykernel.comm.Comm` class has been deprecated. Please use the `comm` module instead.For creating comms, use the function `from comm import create_comm`.\n  comm = Comm('this-comm-tests-a-missing-handler', data={'id': 'foo'})\n"}]
E             Right contains one more item: {'name': 'stderr', 'output_type': 'stream', 'text': "<IPY-INPUT>:1: DeprecationWarning: The `ipykernel.comm.Comm` clas...he function `from comm import create_comm`.\n  comm = Comm('this-comm-tests-a-missing-handler', data={'id': 'foo'})\n"}
E             Full diff:
E               [
E             +  ,
E             -  {'name': 'stderr',
E             -   'output_type': 'stream',
E             -   'text': '<IPY-INPUT>:1: DeprecationWarning: The `ipykernel.comm.Comm` class '
E             -           'has been deprecated. Please use the `comm` module instead.For '
E             -           'creating comms, use the function `from comm import create_comm`.\n'
E             -           "  comm = Comm('this-comm-tests-a-missing-handler', data={'id': "
E             -           "'foo'})\n"},
E               ]

nbclient/tests/test_client.py:289: AssertionError

[blink1073]

I'll release a minor version on Monday.