Drop Python 3.6

[mwtoews]

According to PEP 494, Python 3.6 will reach it's EOL in December 2021. I'd like to suggest removing support for Python 3.6.

The CI tests should also start testing Python 3.9, which has been out for nearly a year already.

[snowman2]

Python 3.10 is coming in October ref. Also, it might not be bad to also consider dropping support for Python 3.7.

Community references:

• NEP-29 says numpy will drop support of Python 3.7 in December 2021. pandas/xarray will likely follow suit.
• opendatacube is Python 3.8+ already ref
• pyproj will drop support soon ref

[mwtoews]

Closed by b4dcc40

See also #1079 (comment) which may draw support for dropping Python 3.7 for Fiona 1.9, depending on release timeline.