Add tox.ini file

Currently, when testing codespell on a local system, it is necessary to
first install the package manually.

Add a tox file to improve user experience and ensure each step runs in
an isolated environment.

There are two changes, compared to the current workflow:
  - pytest runs with coverage enabled
  - the package is not installed in editable mode

Additionally, for convenience, there is a step testing the dictionaries.

Update .coveragerc to exclude the .tox directory.

tox.ini

@@ -0,0 +1,19 @@
+[tox]
+min_version = 4.0
+env_list = unitest,type,dictionaries
+
+[testenv:unitest]
+description = run unit tests
+extras = dev
+commands = pytest --cov=codespell_lib codespell_lib
+
+[testenv:type]
+description = run type checks
+extras = types
+commands = mypy codespell_lib
+
+[testenv:dictionaries]
+description = run dictionary checks
+skip_install = true
+allowlist_externals = make
+commands = make check-dictionaries