Support Python 3.9

[lisphilar]

We need to support the new versions of Python, including Python 3.9.

However, Python 3.9 is not supported in Semaphore CI and it is difficult to run tests with this new version at this time. This was discussed in #243.
https://classic-docs.semaphoreci.com/docs/ubuntu-1804.html

We will reopen issue #243 when Python 3.9 will be supported in Semaphore CI (or new solutions will be found).

[lisphilar]

Python 3.9 was supported with #481 in the latest development version.