Lock filelock for dev-py39 env (#138)

- filelock 3.20.0 and newer dropped support for py39

Author: rbikar

requirements-dev.in

@@ -6,5 +6,6 @@ pytest-cov == 7.0.0
 pytest == 8.4.2
 requests-mock == 1.12.1
 typing-extensions == 4.13.0
+filelock == 3.19.1; python_version == '3.9'
 alabaster == 1.0.0; python_version >= '3.10'
 sphinx == 8.2.3; python_version >= '3.10'