Replace skipsdist with skip_install

The use of the `skipsdist` global setting may have some undesired side
effects, let's drop it in favor of the upstream recommendations.

References:
https://tox.wiki/en/latest/config.html#skip_install
tox-dev/tox#2730
https://review.opendev.org/c/x/charm-ovn-central/+/869165/comments/9e2cf4c4_acea15b6

global/classic-zaza/tox.ini

@@ -9,13 +9,16 @@
 #       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
 
 [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}

global/source-zaza/src/tox.ini

@@ -6,13 +6,16 @@
 
 [tox]
 envlist = pep8
-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
 
 [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
 allowlist_externals = juju

global/source-zaza/tox.ini

@@ -5,14 +5,17 @@
 #     https://github.com/openstack-charmers/release-tools
 
 [tox]
-skipsdist = True
 envlist = pep8,py3
 # 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
 
 [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
          TERM=linux