Modernize tox configuration

The changes modernize the tox configuration by:
- Bumping minimum tox version from 3.1.1 to 3.18.0
- Replacing deprecated whitelist_externals with allowlist_externals
- Enabling usedevelop = True for development mode
- Adding proper environment variable pass-through with documentation
- Updating the constraints file URL to the newer releases.openstack.org location
- Improving the overall structure and formatting
- Use 2 space indentation

Change-Id: Iaee4242749d8549f947ee5c1876b663128b0d515
Signed-off-by: Chandan Kumar (raukadah) <chkumar@redhat.com>
Co-Authored-By: Sean Mooney <work@seanmooney.info>

tox.ini

@@ -1,22 +1,33 @@
 [tox]
-minversion = 3.1.1
+minversion = 3.18.0
 envlist = py3,pep8
-skipsdist = True
-ignore_basepython_conflict = True
 
 [testenv]
-basepython = python3
-usedevelop = False
-whitelist_externals = rm
-install_command = pip install -c{env:TOX_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}
+usedevelop = True
+allowlist_externals = rm
 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
+  VIRTUAL_ENV={envdir}
+  PYTHONWARNINGS=default::DeprecationWarning
+  OS_STDOUT_CAPTURE=1
+  OS_STDERR_CAPTURE=1
+  OS_TEST_TIMEOUT=60
+  PYTHONDONTWRITEBYTECODE=1
+passenv =
+  # LOCALE_ARCHIVE is needed when not using python from the system package
+  # manager (e.g. when installed with nix) to ensure the locale-archive is
+  # available to the tests. specifically this is needed by sphinx to build
+  # the docs.
+  LOCALE_ARCHIVE
+  OS_DEBUG
+  # NOTE(sean-k-mooney) optimization is enabled by default and when enabled
+  # asserts are complied out. Disable optimization to allow asserts in
+  # cyborgclient to fire in unit and functional tests. This can be useful
+  # for debugging issue with fixtures and mocks.
+  PYTHONOPTIMIZE
+deps =
+  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -r{toxinidir}/requirements.txt
+  -r{toxinidir}/test-requirements.txt
 commands = stestr run {posargs}
 
 [testenv:pep8]
@@ -27,17 +38,18 @@ commands = {posargs}
 
 [testenv:cover]
 setenv =
-    VIRTUAL_ENV={envdir}
-    PYTHON=coverage run --source cyborgclient --parallel-mode
+  VIRTUAL_ENV={envdir}
+  PYTHON=coverage run --source cyborgclient --parallel-mode
 commands =
-    stestr run {posargs}
-    coverage combine
-    coverage html -d cover
-    coverage xml -o cover/coverage.xml
+  stestr run {posargs}
+  coverage combine
+  coverage html -d cover
+  coverage xml -o cover/coverage.xml
 
 [testenv:docs]
-deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
-       -r{toxinidir}/doc/requirements.txt
+deps =
+  -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
+  -r{toxinidir}/doc/requirements.txt
 commands = sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:releasenotes]