PYTHON-2262 Test Python 3.9 in Evergreen

[ShaneHarvey]

This change adds testing for Python 3.9 and fixes:

[2020/09/10 20:55:18.185] FAIL [0.002s]: test_instantiation (test_custom_types.TestBSONTypeEnDeCodecs)
[2020/09/10 20:55:18.185] ----------------------------------------------------------------------
[2020/09/10 20:55:18.185] TypeError: Can't instantiate abstract class testcodec with abstract method transform_python
[2020/09/10 20:55:18.185] During handling of the above exception, another exception occurred:
[2020/09/10 20:55:18.185] Traceback (most recent call last):
[2020/09/10 20:55:18.185]   File "/data/mci/183724bc86e179ad6d136f6b74916c26/src/test/test_custom_types.py", line 270, in test_instantiation
[2020/09/10 20:55:18.185]     run_test(TypeEncoder, {'python_type': MyType,}, fail=True)
[2020/09/10 20:55:18.185]   File "/data/mci/183724bc86e179ad6d136f6b74916c26/src/test/test_custom_types.py", line 263, in run_test
[2020/09/10 20:55:18.185]     codec()
[2020/09/10 20:55:18.185] AssertionError: "Can't instantiate abstract class .* with abstract methods .*" does not match "Can't instantiate abstract class testcodec with abstract method transform_python"

I've opened https://jira.mongodb.org/browse/PYTHON-2365 and https://jira.mongodb.org/browse/PYTHON-2366 as follow up work to test snappy, FLE, and OCSP. You'll notice this is also based on #484 to get the tests green.

[prashantmital]

Instead of outright removing the latter half of this regex, might it be acceptable to use the following instead?
Can't instantiate abstract class .* with abstract method.* .*

[ShaneHarvey]

Isn't Can't instantiate abstract class enough to ensure the TypeError is from instantiation?

[prashantmital]

👍

[prashantmital]

Thanks for adding this!

[prashantmital]

👍

[ShaneHarvey]

Rebased to remove the "PYTHON-2362 Use dnspython<2.0 to avoid timeouts" commit which was already merged.