diff --git a/mypy.ini b/mypy.ini
index 02b0e9f..35e2cdd 100644
--- a/mypy.ini
+++ b/mypy.ini
@@ -1,24 +1,27 @@
 [mypy]
 namespace_packages = True
 warn_unused_ignores = True
 
 
 # 3rd party libraries without stubs (yet)
 
 [mypy-confluent_kafka.*]
 ignore_missing_imports = True
 
 [mypy-elasticsearch.*]
 ignore_missing_imports = True
 
 [mypy-msgpack.*]
 ignore_missing_imports = True
 
 [mypy-pkg_resources.*]
 ignore_missing_imports = True
 
 [mypy-pytest.*]
 ignore_missing_imports = True
 
 [mypy-tree_sitter.*]
 ignore_missing_imports = True
+
+[mypy-iso8601.*]
+ignore_missing_imports = True
diff --git a/requirements.txt b/requirements.txt
index 422247f..4c08657 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,7 +1,8 @@
 # Add here external Python modules dependencies, one per line. Module names
 # should match https://pypi.python.org/pypi names. For the full spec or
 # dependency lines, see https://pip.readthedocs.org/en/1.1/requirements.html
 click
 elasticsearch>=7.0.0,<8.0.0
 typing-extensions
 tree_sitter
+iso8601
diff --git a/swh/__init__.py b/swh/__init__.py
index 030e022..b36383a 100644
--- a/swh/__init__.py
+++ b/swh/__init__.py
@@ -1,9 +1,3 @@
-# Copyright (C) 2019-2021  The Software Heritage developers
-# See the AUTHORS file at the top-level directory of this distribution
-# License: GNU General Public License version 3, or any later version
-# See top-level LICENSE file for more information
-
 from pkgutil import extend_path
-from typing import List
 
-__path__: List[str] = extend_path(__path__, __name__)
+__path__ = extend_path(__path__, __name__)
diff --git a/tox.ini b/tox.ini
index b70d51c..19ff8b2 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,74 +1,74 @@
 [tox]
 envlist=black,flake8,mypy,py3
 
 [testenv]
 passenv = YARN
 extras =
   testing
 deps =
   pytest-cov
 commands =
   pytest --doctest-modules \
          {envsitepackagesdir}/swh/search \
          --cov={envsitepackagesdir}/swh/search \
          --cov-branch {posargs}
 
 [testenv:black]
 skip_install = true
 deps =
   black==19.10b0
 commands =
   {envpython} -m black --check swh
 
 [testenv:flake8]
 skip_install = true
 deps =
   flake8
 commands =
   {envpython} -m flake8
 
 [testenv:mypy]
 extras =
   testing
 deps =
-  mypy
+  mypy==0.920
 commands =
   mypy swh
 
 # build documentation outside swh-environment using the current
 # git HEAD of swh-docs, is executed on CI for each diff to prevent
 # breaking doc build
 [testenv:sphinx]
 whitelist_externals = make
 usedevelop = true
 extras =
   testing
 deps =
   # fetch and install swh-docs in develop mode
   -e git+https://forge.softwareheritage.org/source/swh-docs#egg=swh.docs
 
 setenv =
   SWH_PACKAGE_DOC_TOX_BUILD = 1
   # turn warnings into errors
   SPHINXOPTS = -W
 commands =
   make -I ../.tox/sphinx/src/swh-docs/swh/ -C docs
 
 
 # build documentation only inside swh-environment using local state
 # of swh-docs package
 [testenv:sphinx-dev]
 whitelist_externals = make
 usedevelop = true
 extras =
   testing
 deps =
   # install swh-docs in develop mode
   -e ../swh-docs
 
 setenv =
   SWH_PACKAGE_DOC_TOX_BUILD = 1
   # turn warnings into errors
   SPHINXOPTS = -W
 commands =
   make -I ../.tox/sphinx-dev/src/swh-docs/swh/ -C docs