Test all Python versions currently in support. (#56)

* Test all Python versions currently in support.

* Update CHANGELOG

Author: bgrant

.travis.yml

@@ -2,6 +2,8 @@ language: python
 python:
   - '3.6'
   - '3.7'
+  - '3.8'
+  - '3.9'
 install:
   - make install
 script:

CHANGELOG.md

@@ -13,6 +13,7 @@ Versioning](https://semver.org/spec/v2.0.0.html).
 - [PR 52](https://github.com/salesforce/django-declarative-apis/pull/52) Update cryptography and coverage dependencies
 - [PR 53](https://github.com/salesforce/django-declarative-apis/pull/53) Update dev dependencies: flake8, bandit, ipython
 - [PR 54](https://github.com/salesforce/django-declarative-apis/pull/54) Remove unneeded `mock` dependency
+- [PR 56](https://github.com/salesforce/django-declarative-apis/pull/56) Add Python 3.8 and 3.9 to Travis CI testing
 
 # [0.22.2] - 2020-08-11
 ### Fixed