[stable-2.7] Limit tests to paramiko < 2.5.0.

(cherry picked from commit 1e6edf2ccc)

Co-authored-by: Matt Clay <matt@mystile.com>

test/runner/requirements/constraints.txt

@@ -15,6 +15,7 @@ pycrypto >= 2.6 # Need features found in 2.6 and greater
 ncclient >= 0.5.2 # Need features added in 0.5.2 and greater
 idna < 2.6 # requests requires idna < 2.6, but cryptography will cause the latest version to be installed instead
 paramiko < 2.4.0 ; python_version < '2.7' # paramiko 2.4.0 drops support for python 2.6
+paramiko < 2.5.0 ; python_version >= '2.7' # paramiko 2.5.0 requires cryptography 2.5.0+
 pytest < 3.3.0 ; python_version < '2.7' # pytest 3.3.0 drops support for python 2.6
 ntlm-auth >= 1.0.6 # message encryption support
 requests < 2.20.0 ; python_version < '2.7' # requests 2.20.0 drops support for python 2.6