Cleanup tox/travis

* Switch to python 3.8 in Travis. I tried to add 3.8 while keeping 3.7
and 3.6, but ran into issues with Travis config, so instead just bumped
straight to 3.8. Long term I'd like to explore moving to Azure
Pipelines, but don't have the time to figure that out just yet.
* `flake8` was renamed to `pycodestyle`
* `py.test` was deprecated in favor of `pytest`

.travis.yml

@@ -1,9 +1,9 @@
 sudo: false
 language: python
-python: "3.6"
+python: "3.8"
 env:
   - TOXENV=py27
-  - TOXENV=py36
+  - TOXENV=py38
 install:
   - pip install tox
 script: tox

tox.ini

@@ -1,19 +1,19 @@
 [tox]
-envlist = py27,py36,stylecheck
+envlist = py27,py36,py37,py38,stylecheck
 
 [testenv]
 deps =
     pytest
     Flask-SQLAlchemy
     Pygments
 commands =
-    py.test
+    pytest
 
 [testenv:stylecheck]
 deps =
-    flake8
+    pycodestyle
 commands =
-    flake8 flask_debugtoolbar test
+    pycodestyle flask_debugtoolbar test
 
-[flake8]
+[pycodestyle]
 max-line-length = 100