gh_actions: add initial test workflow

The intial test uses the python-lint action to validate code style
according to the preferences used for Python code in the Moonraker
repo.

Signed-off-by:  Eric Callahan <arksine.code@gmail.com>

.github/workflows/test_code.yaml

@@ -0,0 +1,17 @@
+# CI for code style and application tests
+
+name: test-code
+on: [push, pull_request]
+jobs:
+    lint-python-code:
+    - uses: actions/checkout@v1
+    - uses: ricardochaves/python-lint@v1.4.0
+        with:
+        python-root-list: "moonraker scripts"
+        use-pylint: false
+        use-pycodestyle: true
+        use-flake8: false
+        use-black: false
+        use-mypy: false
+        use-isort: false
+        extra-pycodestyle-options: "--ignore=E226,E301,E302,E303,W503,W504 --max-line-length=80 --max-doc-length=80"