Add HTTP/2 tempest scenario tests for listeners

Depends-On: https://review.opendev.org/#/c/744905/
Depends-On: https://review.opendev.org/#/c/747932/
Change-Id: I90594ab948c74711682c9bb14d37d1e1d7b1beeb
diff --git a/requirements.txt b/requirements.txt
index e5b93b8..b18679b 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -18,3 +18,4 @@
 tenacity>=4.4.0 # Apache-2.0
 testtools>=2.2.0 # MIT
 keystoneauth1>=3.3.0 # Apache-2.0
+httpx[http2]>=0.14.2 # BSD