Install PBR explicitly

We do this because easy_install on python3.5 is failing to find pbr. I
believe this is happening due to pypi's deprecation of non SNI tls
connections. They report that the version of 3.5 on xenial should be
able to do SNI but easy_install at least doesn't seem to do so.

Note we restrict this to python>=3 and <=3.5 as python 2.7 had
backported fixes. Also, this really only helps with unittesting via tox
as we instruct tox to explicitly install the requirements files before
creating/installing the package itself. This is the reason we stick this
in test-requirements.txt rather than requirements.txt.

Change-Id: Iac2c1f4f575de250ed0dba22d33b017322fd7562

test-requirements.txt

@@ -1,3 +1,7 @@
+# This is listed in order to preinstall PBR with pip as easy_install
+# on these older python versions can no longer reliably talk to pypi
+# due to non SNI TLS connections being disabled.
+pbr ; python_version >= "3" and python_version <= "3.5"
 hacking>=2.0.0,<2.1.0
 mock
 fixtures>=0.3.14

tox.ini

@@ -1,9 +1,15 @@
 [tox]
 envlist = linters,docs,py3
 ignore_basepython_conflict = true
+# We skipsdist to force test-requirements and specifically pbr
+# to be installed prior to installing git-review. This ensures that
+# easy_install doesn't try to talk to pypi.org for pbr which fails
+# on older python due to a lack of SNI support.
+skipsdist = true
 
 [testenv]
 basepython = python3
+usedevelop = true
 # See "testing behind a proxy" https://review.opendev.org/624496
 passenv =
   http_proxy