tox.ini: Fix Python environment factors

Tox's default nomenclature for referring to Python test environments
(without further config options) follows the 'py3[[:digit:]]*'
expression, in other words if one wants to run tests against Python
3.9, they'd put 'py39' in the 'envlist' setting, similarly for py310,
py311, etc.

We didn't follow the rules and so we always executed tox against the
Python version that was originally used to create the development
virtual environment. This is proved by the tox's output which reports:

    python3.10: install_deps> python -I -m pip install -r
        requirements-extras.txt
    ...
    ===========================================================
    platform linux -- Python 3.11.5, pytest-7.4.2, pluggy-1.3.0

despite having been instructed (though in an incorrect way) to run a
Python 3.10 environment. This behaviour also silently ignored cases
where the given Python version wasn't even installed on the system, so
the platform Python which was used to create the virtual environment
was always used.

After this patch, this is what the execution reports:

    py310: install_deps> python -I -m pip install -r
        requirements-extras.txt
    .pkg-cpython310: install_requires> python -I -m pip install
        setuptools setuptools-scm
    ...
    ============================================================
    platform linux -- Python 3.10.13, pytest-7.4.2, pluggy-1.3.0

AND

    py310: skipped because could not find python interpreter with
        spec(s): py310

if python3.10 isn't installed on the host system.

While at it, use Bash-like brace expansion to cover a range of minor
Python 3 versions instead of explicitly enumerating each one of them.

Signed-off-by: Erik Skultety <[email protected]>

Makefile

@@ -1,5 +1,5 @@
 PYTHON_VERSION_VENV ?= python3.9
-TOX_ENVLIST ?= python3.9
+TOX_ENVLIST ?= py39
 TOX_ARGS ?=
 
 all: venv

README.md

@@ -197,21 +197,21 @@ For finer control over which tests get executed, e.g. to run all tests in a spec
 the [virtualenv](#virtualenv) and run:
 
 ```shell
-tox -e python3.9 -- tests/unit/test_cli.py
+tox -e py39 -- tests/unit/test_cli.py
 ```
 
 Even better, run it stepwise (exit on first failure, re-start from the failed test next time):
 
 ```shell
-tox -e python3.9 -- tests/unit/test_cli.py --stepwise
+tox -e py39 -- tests/unit/test_cli.py --stepwise
 ```
 
 You can also run a single test class or a single test method:
 
 ```shell
-tox -e python3.9 -- tests/unit/test_cli.py::TestGenerateEnv
-tox -e python3.9 -- tests/unit/test_cli.py::TestGenerateEnv::test_invalid_format
-tox -e python3.9 -- tests/unit/extras/test_envfile.py::test_cannot_determine_format
+tox -e py39 -- tests/unit/test_cli.py::TestGenerateEnv
+tox -e py39 -- tests/unit/test_cli.py::TestGenerateEnv::test_invalid_format
+tox -e py39 -- tests/unit/extras/test_envfile.py::test_cannot_determine_format
 ```
 
 In short, tox passes all arguments to the right of `--` directly to pytest.

tox.ini

@@ -7,9 +7,7 @@ envlist =
     black
     isort
     mypy
-    python3.9
-    python3.10
-    python3.11
+    py3{9,10,11}
 
 [testenv]
 passenv =
@@ -74,9 +72,9 @@ skipsdist = true
 
 [gh-actions]
 python =
-    3.9: python3.9
-    3.10: python3.10
-    3.11: python3.11
+    3.9: py39
+    3.10: py310
+    3.11: py311
 
 [flake8]
 show-source = True