Migrate to pysnmp lextudio ecosystem

The pysnmp library is not maintained since 4 years now and it's
incompatible with recent libraries like pyasn1.
Its fork pysnmp-lextudio is regularly maintained, we should move
to that.
For more info see https://github.com/etingof/pysnmp/issues/429

Change-Id: Id6dbf28d69e60465e7c8a71d6afbe227378ee60a

.zuul.yaml

@@ -1,5 +1,5 @@
 - project:
     templates:
       - check-requirements
-      - openstack-python3-yoga-jobs
+      - openstack-python3-jobs
       - publish-openstack-docs-pti

requirements.txt

@@ -4,7 +4,9 @@
 pbr!=2.1.0,>=2.0.0 # Apache-2.0
 Babel!=2.4.0,>=2.3.4 # BSD
 pyghmi>=1.0.24 # Apache-2.0
-pysnmp>=4.2.3 # BSD
+pysnmp-lextudio>=5.0.0 # BSD
+pyasn1-lextudio>=1.1.0 # BSD
+pyasn1-modules-lextudio>=0.2.0 # BSD
 requests>=2.14.2 # Apache-2.0
 defusedxml>=0.7.0 # PSF
 six>=1.10.0 # MIT

test-requirements.txt

@@ -5,8 +5,7 @@
 coverage!=4.4,>=4.0 # Apache-2.0
 fixtures>=3.0.0 # Apache-2.0/BSD
 python-subunit>=1.0.0 # Apache-2.0/BSD
-oslotest>=3.2.0,<4;python_version<'3.6'  # Apache-2.0
-oslotest>=3.2.0;python_version>='3.6'  # Apache-2.0
+oslotest>=3.2.0 # Apache-2.0
 stestr>=2.0.0,!=2.3.0,!=3.0.0  # Apache-2.0
 testscenarios>=0.4 # Apache-2.0/BSD
 testtools>=2.2.0 # MIT