Stop configuring install_command in tox.

Currently, we are overriding 'install_command' to use 'pip'. This is
considered poor behavior and 'python -m pip' should be used instead:

https://snarky.ca/why-you-should-use-python-m-pip/

It turns out that this is the the default value provided by tox:

https://tox.readthedocs.io/en/latest/config.html#conf-install_command

So we can remove the line and simply use the default value.

openstack-discuss thread
http://lists.openstack.org/pipermail/openstack-discuss/2020-April/014237.html

Bumping pbr since oslo.log requires 3.1.1

Change-Id: I1f968c724e79e04bbc2fe8cb26fd54404b0350e0

lower-constraints.txt

@@ -59,7 +59,7 @@ oslo.utils==4.1.0
 oslotest==3.2.0
 Paste==3.4.0
 PasteDeploy==2.1.0
-pbr==2.0.0
+pbr==3.1.1
 prettytable==0.7.2
 prometheus-client==0.6.0
 pycodestyle==2.5.0

requirements.txt

@@ -1,4 +1,4 @@
-pbr!=2.1.0,>=2.0.0 # Apache-2.0
+pbr>=3.1.1 # Apache-2.0
 stevedore>=1.20.0 # Apache-2.0
 oslo.messaging>=9.4.0 # Apache-2.0
 Flask>=1.0.0 # BSD

tox.ini

@@ -7,7 +7,6 @@ ignore_basepython_conflict=true
 [testenv]
 basepython = python3
 usedevelop = True
-install_command = pip install {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
    PYTHONDONTWRITEBYTECODE = 1