[Tests] Update tox and CI configs

- Add new requirements files to make it easier to install deps.

- Tox changes
  - Update tox to use new requirements files.
  - Tweak heading styles.
  - Add development environment command `devenv`.
  - Remove testenv command as it would run on devenv creation.

- Travis changes
  - Now uses xenial as trusty is very old now.
  - Trial run disabled to speed up tests.
  - Add tox-venv for Python 3 support.
  - Only install testssl if running security tests.

- Appveyor
  - Add tox-venv for Python 3 support.
  - Use requirements file for non-tox.
  - Remove trial run to speed up testing.

requirements-dev.txt

@@ -0,0 +1,16 @@
+-r requirements.txt
+-r requirements-docs.txt
+-r requirements-tests.txt
+sphinx-autobuild
+tox
+tox-venv
+detox
+pre-commit
+pre-commit-hooks
+flake8-blind-except
+flake8-builtins
+flake8-commas
+flake8-comprehensions
+flake8-debugger
+flake8-mock
+flake8-mutable