tests: Tighten requirements.txt, due to new package releases

Attempt to fix

Collecting hdrhistogram==0.6.1 (from -r tests/ansible/requirements.txt
(line 2))
Downloading
2422ad2ad90047d3b9f73061e8696f/hdrhistogram-0.6.1.tar.gz
(61kB)
    Complete output from command python setup.py egg_info:
    Couldn't find index page for 'pbr' (maybe misspelled?)
        sources=['src/python-codec.c'])]
      File "/usr/lib/python3.5/distutils/core.py", line 108, in setup
        _setup_distribution = dist = klass(attrs)
File "/tmp/venv/lib/python3.5/site-packages/setuptools/dist.py",
line 315, in __init__
        self.fetch_build_eggs(attrs['setup_requires'])
File "/tmp/venv/lib/python3.5/site-packages/setuptools/dist.py",
line 361, in fetch_build_eggs
        replace_conflicting=True,
File
"/tmp/venv/lib/python3.5/site-packages/pkg_resources/__init__.py", line
850, in resolve
        dist = best[req.key] = env.best_match(req, ws, installer)
File
"/tmp/venv/lib/python3.5/site-packages/pkg_resources/__init__.py", line
1122, in best_match
        return self.obtain(req, installer)
File
"/tmp/venv/lib/python3.5/site-packages/pkg_resources/__init__.py", line
1134, in obtain
        return installer(requirement)
File "/tmp/venv/lib/python3.5/site-packages/setuptools/dist.py",
line 429, in fetch_build_egg
        return cmd.easy_install(req)
File
"/tmp/venv/lib/python3.5/site-packages/setuptools/command/easy_install.py",
line 659, in easy_install
        raise DistutilsError(msg)
distutils.errors.DistutilsError: Could not find suitable
distribution for Requirement.parse('pbr')

.ci/prep_azure.py

@@ -84,6 +84,12 @@ else:
 if need_to_fix_psycopg2:
     venv_steps.append('/tmp/venv/bin/pip3 install psycopg2==2.8.5 psycopg2-binary')
 
+venv_steps.extend([
+    # pbr is a transitive setup_requires of hdrhistogram. If it's not already
+    # installed then setuptools attempts to use easy_install, which fails.
+    '/tmp/venv/bin/pip install pbr==5.6.0',
+])
+
 batches.append(venv_steps)
 
 ci_lib.run_batches(batches)

tests/requirements.txt

@@ -12,6 +12,7 @@ timeoutcontext==1.2.0
 unittest2==1.1.0
 # Fix InsecurePlatformWarning while creating py26 tox environment
 # https://urllib3.readthedocs.io/en/latest/advanced-usage.html#ssl-warnings
-urllib3[secure]; python_version < '2.7.9'
+urllib3[secure]==1.23; python_version < '2.7'
+urllib3[secure]==1.26; python_version > '2.6' and python_version < '2.7.9'
 # Last idna compatible with Python 2.6 was idna 2.7.
 idna==2.7; python_version < '2.7'