Cap paramiko to releases below 2.5.0

This new version makes the kayobe-tox-molecule job fail with:

    ImportError: cannot import name py31compat

Change-Id: Ieddf039483649cc7b575019409257b6ce5b3fb99

requirements.txt

@@ -10,5 +10,7 @@ pbr>=2.0 # Apache-2.0
 ansible>=2.4.0,<2.7.0 # GPLv3
 cliff>=2.5.0 # Apache
 netaddr!=0.7.16,>=0.7.13 # BSD
+# paramiko 2.5.0 makes the kayobe-tox-molecule job fail.
+paramiko<2.5.0
 PyYAML>=3.10.0 # MIT
 setuptools!=24.0.0,!=34.0.0,!=34.0.1,!=34.0.2,!=34.0.3,!=34.1.0,!=34.1.1,!=34.2.0,!=34.3.0,!=34.3.1,>=16.0 # PSF/ZPL