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.

(cherry picked from commit b3a3e8ba7b)

test/runner/requirements/constraints.txt

@@ -17,3 +17,6 @@ ntlm-auth >= 1.0.6 # message encryption support
 requests-ntlm >= 1.1.0 # message encryption support
 requests-credssp >= 0.1.0 # message encryption support
 voluptuous >= 0.11.0 # Schema recursion via Self
+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