Add typing stubs dependencies for mypy>0.900

requirements-test.txt

 pytest
 requests  # workaround for https://forge.softwareheritage.org/T2634
 requests-mock
+
+types-python-dateutil
+types-pyyaml
+types-requests

requirements.txt

-PyYAML
+pyyaml
 aiosqlite
 psutil
 pyfuse3

swh/__init__.py

 from pkgutil import extend_path
-from typing import Iterable
+from typing import List
 
-__path__: Iterable[str] = extend_path(__path__, __name__)
+__path__: List[str] = extend_path(__path__, __name__)