Add a testenv for lower-constraints

Give projects a testenv for testing lower constraints by default, as
suggested by Tony in the recent Requirements project update in Denver:

    https://youtu.be/55ZlXiuiLCo?t=159

and by Dirk:

    https://review.opendev.org/#/c/657444/2/%257B%257Bcookiecutter.repo_name%257D%257D/tox.ini@8

Change-Id: I2456a477ab3f36d2b63420c502bc5409770a4712

{{cookiecutter.repo_name}}/lower-constraints.txt

@@ -0,0 +1,4 @@
+# You need to rewrite this file with the project's lower constraints.
+# This can be done using the following script:
+#
+# https://opendev.org/openstack/requirements/src/branch/master/tools/fix-lower-constraints.py

{{cookiecutter.repo_name}}/tox.ini

@@ -16,6 +16,11 @@ deps = -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/
        -r{toxinidir}/test-requirements.txt
 commands = stestr run {posargs}
 
+[testenv:lower-constraints]
+basepython = python3
+deps = -c{toxinidir}/lower-constraints.txt
+       -r{toxinidir}/test-requirements.txt
+
 [testenv:pep8]
 commands = flake8 {posargs}