Drop support for py36 and add py39 and py310

[hpesonen]

Summary:

Update test matrix by dropping support for py36 and adding py39 and py310

Please make sure

• [x] You have updated the CHANGELOG.rst
• [x] You have provided a short summary of your changes (see previous section)
• [x] You have listed the copyright holder for the work you are submitting (see next section)

If your contribution adds, removes or somehow changes the functional behavior of the package, please check that

• [ ] You have included or updated all the relevant documentation
• [ ] You have added appropriate unit tests to ensure the new features behave as expected

and the proposed changes pass all unit tests (check step 6 of CONTRIBUTING.rst for details)

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company):

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD3 (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[uremes]

python 3.6 is still mentioned in CONTRIBUTING.rst, installation.rst, quickstart.rst, and setup.py

[hpesonen]

python 3.6 is still mentioned in CONTRIBUTING.rst, installation.rst, quickstart.rst, and setup.py

Good point, made the changes in CONTRIBUTING.rst installation.rst setup.py

quickstart.rst is generated from the notebooks, I'll update them in notebooks PR and update quickstart.rst after that.