requirements-test: Restrict hypothesis <6

This fixes build [1]

[1] https://jenkins.softwareheritage.org/view/swh%20master/job/DCORE/job/tests/1183/console

requirements-test.txt

 pytest
 pytest-mock
 requests-mock
-hypothesis >= 3.11.0
+hypothesis >= 3.11.0, < 6
 pre-commit
 pytz