Add 3.9 and 3.10.1 to travis build (#239)

* Add 3.9 and 3.10 to travis build

* Replace 3.10 with 3.10-dev

* Update travis to use Ubuntu 18.04

18.04 has 3.10.0 and 3.10.1 according to: 
https://docs.travis-ci.com/user/languages/python/

* Bump python 3.10 to 3.10.1 in travis

* Unpin pytest version

Travis build for 3.10.1 failed because of issue in pytest that was fixed in a later release: pytest-dev/pytest#8540

.travis.yml

@@ -1,9 +1,12 @@
 language: python
+dist: bionic  # Ubuntu 18.04
 python:
   - '3.5'
   - '3.6'
   - '3.7'
   - '3.8'
+  - '3.9'
+  - '3.10.1'
 before_script:
   - pip install tox
   - if [[ $TRAVIS_PYTHON_VERSION == 3.6 ]]; then pip install flake8 flake8-bugbear; fi

setup.py

@@ -28,7 +28,7 @@ def read(filename):
     # Twisted 19.7.0 dropped py3.4 support
     "twisted:python_version == '3.4'": "twisted<=19.2.1",
     "typing": ["typing>=3.6.4"],
-    "tests": ["pytest==4.6.5", "pytest-mock", "pytest-cov", "pytest-twisted"],
+    "tests": ["pytest", "pytest-mock", "pytest-cov", "pytest-twisted"],
 }
 
 metadata = {