developer-setup: Mention the current mypy version we use

To avoid discrepancy with tox when pre-commit is triggered.

Related to swh/infra/sysadm-environment#3746 (closed)

---

Migrated from D7054 (view on Phabricator)

docs/developer-setup.rst

@@ -94,6 +94,10 @@ Checkout all the swh packages source repositories::
     (swh) ~/swh-environment$ pip install pre-commit
     (swh) ~/swh-environment$ ./bin/update
 
+Use the same mypy version our tox containers use:
+
+    (swh) ~/swh-environment$ pip install mypy==0.920
+
 In the future you can re-activate the created virtualenv with::
 
    $ workon swh