Drop install_command usage in tox

Use the default value provided by tox:

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

See discussion on the openstack-discuss ML [1]
for the complete context.

[1] http://lists.openstack.org/pipermail/openstack-discuss/2020-April/014237.html

Change-Id: I93c60668c9510f4479d268aab69d0117a53b2c01
Signed-off-by: Goutham Pacha Ravi <gouthampravi@gmail.com>
diff --git a/tox.ini b/tox.ini
index 59764db..68f864e 100644
--- a/tox.ini
+++ b/tox.ini
@@ -6,15 +6,17 @@
 [testenv]
 basepython = python3
 usedevelop = True
-install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master} {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
    PYTHONWARNINGS=default::DeprecationWarning
    OS_STDOUT_CAPTURE=1
    OS_STDERR_CAPTURE=1
    OS_TEST_TIMEOUT=60
-deps = -r{toxinidir}/requirements.txt
-       -r{toxinidir}/test-requirements.txt
+deps =
+  -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -r{toxinidir}/requirements.txt
+  -r{toxinidir}/test-requirements.txt
+
 # commands = python setup.py test --slowest --testr-args='{posargs}'
 
 [testenv:pep8]