Add constraint for requests on python 2.6. (#47306)

* Add constraint for requests on python 2.6.
* Use constraints in lookup_hashi_vault test.

(cherry picked from commit f74f7b0373)

test/integration/targets/lookup_hashi_vault/playbooks/install_dependencies.yml

@@ -17,3 +17,4 @@
     - name: 'Install hvac Python package'
       pip:
         name: "{{ hvac_package|default('hvac') }}"
+        extra_args: "-c {{ playbook_dir }}/../../../../runner/requirements/constraints.txt"

test/runner/requirements/constraints.txt

@@ -15,6 +15,7 @@ idna < 2.6 # requests requires idna < 2.6, but cryptography will cause the lates
 paramiko < 2.4.0 ; python_version < '2.7' # paramiko 2.4.0 drops support for python 2.6
 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
 requests-ntlm >= 1.1.0 # message encryption support
 requests-credssp >= 0.1.0 # message encryption support
 voluptuous >= 0.11.0 # Schema recursion via Self