Update test constraints. (#44975)

- Limit virtualenv version on Python 2.6.
- Limit pyopenssl version on Python 2.6.
- Pin pyfmg version to keep unit tests passing.

test/runner/requirements/constraints.txt

@@ -18,3 +18,6 @@ requests-ntlm >= 1.1.0 # message encryption support
 requests-credssp >= 0.1.0 # message encryption support
 voluptuous >= 0.11.0 # Schema recursion via Self
 openshift >= 0.6.2 # merge_type support
+virtualenv < 16.0.0 ; python_version < '2.7' # virtualenv 16.0.0 and later require python 2.7 or later
+pyopenssl < 18.0.0 ; python_version < '2.7' # pyOpenSSL 18.0.0 and later require python 2.7 or later
+pyfmg == 0.6.1 # newer versions do not pass current unit tests