Enable tox4 compatibility in its config

This patch adds a workaround for a failure that happens in tox when
`skip_install` is set and `--installpkg` is used.

Ref: tox-dev/tox#2442

Author: webknjaz

tox.ini

@@ -213,6 +213,10 @@ commands =
 [testenv:cleanup-dists]
 description =
   Wipe the the dist{/} folder
+# NOTE: `package_env = none` is needed so it's possible to use `--installpkg`
+# NOTE: with the main `testenv`.
+# Ref: https://github.com/tox-dev/tox/issues/2442
+package_env = ❌ DUMMY NON-EXISTENT ENV NAME ❌
 usedevelop = false
 skip_install = true
 deps =
@@ -248,6 +252,10 @@ install_command =
     '{toxinidir}{/}requirements{/}' \
     install {opts} {packages}
 isolated_build = true
+# NOTE: `package_env = none` is needed so it's possible to use `--installpkg`
+# NOTE: with the main `testenv`.
+# Ref: https://github.com/tox-dev/tox/issues/2442
+package_env = ❌ DUMMY NON-EXISTENT ENV NAME ❌
 # `usedevelop = true` overrides `skip_install` instruction, it's unwanted
 usedevelop = false
 skip_install = true
@@ -280,6 +288,10 @@ depends =
   build-dists
 deps =
   twine
+# NOTE: `package_env = none` is needed so it's possible to use `--installpkg`
+# NOTE: with the main `testenv`.
+# Ref: https://github.com/tox-dev/tox/issues/2442
+package_env = ❌ DUMMY NON-EXISTENT ENV NAME ❌
 usedevelop = false
 skip_install = true
 commands_pre =
@@ -294,6 +306,10 @@ commands =
     {env:PEP517_OUT_DIR}{/}*
 
 [testenv:release]
+# NOTE: `package_env = none` is needed so it's possible to use `--installpkg`
+# NOTE: with the main `testenv`.
+# Ref: https://github.com/tox-dev/tox/issues/2442
+package_env = ❌ DUMMY NON-EXISTENT ENV NAME ❌
 skip_install = True
 deps =
   pep517>=0.5