[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.

pyproject.toml

@@ -2,6 +2,7 @@
 requires = [
     "setuptools",
     "wheel",
+    "slimit",
 ]
 
 [tool.black]