diff --git a/requirements.txt b/requirements.txt
index ead6e89a..3b1cb7b1 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -11,14 +11,19 @@ pbr==5.6.0
 simplejson>=2.2.0
 netifaces>=0.10.4
 
+# NOTE: newer versions of cryptography require a Rust compiler to build,
+# see
+# * https://github.com/openstack-charmers/zaza/issues/421
+# * https://mail.python.org/pipermail/cryptography-dev/2021-January/001003.html
+#
+cryptography<3.4
+
 # Strange import error with newer netaddr:
 netaddr>0.7.16,<0.8.0
 
 Jinja2>=2.6  # BSD License (3 clause)
 six>=1.9.0
 
-# dnspython 2.0.0 dropped py3.5 support
-dnspython<2.0.0; python_version < '3.6'
-dnspython; python_version >= '3.6'
+dnspython
 
 psutil>=1.1.1,<2.0.0
diff --git a/test-requirements.txt b/test-requirements.txt
index 0aabe171..e972406e 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -8,8 +8,6 @@
 #       all of its own requirements and if it doesn't, fix it there.
 #
 pyparsing<3.0.0  # aodhclient is pinned in zaza and needs pyparsing < 3.0.0, but cffi also needs it, so pin here.
-cffi==1.14.6; python_version < '3.6'  # cffi 1.15.0 drops support for py35.
-setuptools<50.0.0  # https://github.com/pypa/setuptools/commit/04e3df22df840c6bb244e9b27bc56750c44b7c85
 
 requests>=2.18.4
 
@@ -19,25 +17,13 @@ stestr>=2.2.0
 # https://github.com/mtreinish/stestr/issues/145
 cliff<3.0.0
 
-# Dependencies of stestr. Newer versions use keywords that didn't exist in
-# python 3.5 yet (e.g. "ModuleNotFoundError")
-importlib-metadata<3.0.0; python_version < '3.6'
-importlib-resources<3.0.0; python_version < '3.6'
-
-# Some Zuul nodes sometimes pull newer versions of these dependencies which
-# dropped support for python 3.5:
-osprofiler<2.7.0;python_version<'3.6'
-stevedore<1.31.0;python_version<'3.6'
-debtcollector<1.22.0;python_version<'3.6'
-oslo.utils<=3.41.0;python_version<'3.6'
-
 coverage>=4.5.2
 pyudev              # for ceph-* charm unit tests (need to fix the ceph-* charm unit tests/mocking)
 git+https://github.com/openstack-charmers/zaza.git#egg=zaza
 git+https://github.com/openstack-charmers/zaza-openstack-tests.git#egg=zaza.openstack
 
 # Needed for charm-glance:
-git+https://opendev.org/openstack/tempest.git#egg=tempest;python_version>='3.6'
-tempest<24.0.0;python_version<'3.6'
+git+https://opendev.org/openstack/tempest.git#egg=tempest
 
 croniter            # needed for charm-rabbitmq-server unit tests
+psutil
diff --git a/tox.ini b/tox.ini
index 2b600f1e..3ea16f60 100644
--- a/tox.ini
+++ b/tox.ini
@@ -9,23 +9,23 @@
 #       all of its own requirements and if it doesn't, fix it there.
 [tox]
 envlist = pep8,py3
-skipsdist = True
 # NOTE: Avoid build/test env pollution by not enabling sitepackages.
 sitepackages = False
 # NOTE: Avoid false positives by not skipping missing interpreters.
 skip_missing_interpreters = False
 
-# NOTE: https://wiki.canonical.com/engineering/OpenStack/InstallLatestToxOnOsci
-minversion = 3.18.0
-
 [testenv]
+# We use tox mainly for virtual environment management for test requirements
+# and do not install the charm code as a Python package into that environment.
+# Ref: https://tox.wiki/en/latest/config.html#skip_install
+skip_install = True
 setenv = VIRTUAL_ENV={envdir}
          PYTHONHASHSEED=0
          CHARM_DIR={envdir}
 commands = stestr run --slowest {posargs}
 allowlist_externals =
     charmcraft
-    {toxinidir}/rename.sh
+    rename.sh
 passenv =
     HOME
     TERM
@@ -37,45 +37,35 @@ deps = -r{toxinidir}/test-requirements.txt
 [testenv:build]
 basepython = python3
 deps = -r{toxinidir}/build-requirements.txt
+# charmcraft clean is done to ensure that
+# `tox -e build` always performs a clean, repeatable build.
+# For faster rebuilds during development,
+# directly run `charmcraft -v pack && ./rename.sh`.
 commands =
     charmcraft clean
-    charmcraft -v build
+    charmcraft -v pack
     {toxinidir}/rename.sh
-
-[testenv:py36]
-basepython = python3.6
-deps = -r{toxinidir}/requirements.txt
-       -r{toxinidir}/test-requirements.txt
-
-[testenv:py37]
-basepython = python3.7
-deps = -r{toxinidir}/requirements.txt
-       -r{toxinidir}/test-requirements.txt
+    charmcraft clean
 
 [testenv:py38]
 basepython = python3.8
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 
-[testenv:py39]
-basepython = python3.9
-deps = -r{toxinidir}/requirements.txt
-       -r{toxinidir}/test-requirements.txt
-
-[testenv:py3]
-basepython = python3
-deps = -r{toxinidir}/requirements.txt
-       -r{toxinidir}/test-requirements.txt
-
 [testenv:py310]
 basepython = python3.10
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 
+[testenv:py3]
+basepython = python3
+deps = -r{toxinidir}/requirements.txt
+       -r{toxinidir}/test-requirements.txt
+
 [testenv:pep8]
 basepython = python3
 deps = flake8==3.9.2
-       charm-tools==2.8.4
+       git+https://github.com/juju/charm-tools.git
 commands = flake8 {posargs} hooks unit_tests tests actions lib files
            charm-proof