🎨 Make the GHA log is clean and colorized

This patch sets up root-level environment variables shared by all the
workflow jobs. They include:

	* Disabling undesired `pip`'s warnings/suggestions
	* Requesting the executed apps color their output unconditionally
	* Letting `tox` pass those requests to underlying/wrapped programs

.github/workflows/main.yml

@@ -2,6 +2,22 @@ name: tests
 
 on: [push, pull_request]
 
+env:
+  FORCE_COLOR: 1  # Request colored output from CLI tools supporting it
+  MYPY_FORCE_COLOR: 1  # MyPy's color enforcement
+  PIP_DISABLE_PIP_VERSION_CHECK: 1
+  PIP_NO_PYTHON_VERSION_WARNING: 1
+  PIP_NO_WARN_SCRIPT_LOCATION: 1
+  PY_COLORS: 1  # Recognized by the `py` package, dependency of `pytest`
+  TOX_PARALLEL_NO_SPINNER: 1
+  TOX_TESTENV_PASSENV: >-  # Make tox-wrapped tools see color requests
+    FORCE_COLOR
+    MYPY_FORCE_COLOR
+    NO_COLOR
+    PY_COLORS
+    PYTEST_THEME
+    PYTEST_THEME_MODE
+
 jobs:
   test:
     strategy: