Run mypy as part of the lint script

CONTRIBUTING.md

@@ -63,10 +63,35 @@ run-time:
 ./scripts-dev/lint.sh path/to/file1.py path/to/file2.py path/to/folder
 ```
 
-You can also provided the `-d` option, which will lint the files that have been
+You can also provide the `-d` option, which will lint the files that have been
 changed since the last git commit. This will often be significantly faster than
 linting the whole codebase.
 
+Another option is to only run types checks, with `mypy`, in a fresh virtual env:
+
+```
+# Create the virtual env, the name of this venv must be 'testenv'
+python -m venv testenv
+
+# activate the virtual env for the current shell and install tox
+source testenv/bin/activate
+pip install tox
+
+# install dependencies and run tests
+tox -e mypy
+```
+
+or quicker, if you just want to run mypy appart from the other tests:
+
+```
+# After creating and activating a venv for the current shell
+# install mypy and the dependencies
+pip install -e .[mypy]
+
+# run tests
+mypy
+```
+
 Before pushing new changes, ensure they don't produce linting errors. Commit any
 files that were corrected.
 

changelog.d/8633.doc

@@ -0,0 +1 @@
+Add instructions to run types checks in an isolated venv.

scripts-dev/lint.sh

@@ -94,3 +94,4 @@ isort "${files[@]}"
 python3 -m black "${files[@]}"
 ./scripts-dev/config-lint.sh
 flake8 "${files[@]}"
+mypy