📦 Pass desired release to PEP 517 tox env

This patch fixes passing the release version string from the release
automation script to the tox's PEP 517 frontend, so that the backend
machinery bits (namely, `setuptools-scm`) could access the environment
variable; as indentified in the attempt to release pytest v9.0.1 [[1]].

The solution uses the `[pkgenv]` section of `tox.ini` that corresponds
to the PEP 517 build env tox manages internally.

[1]: https://github.com/pytest-dev/pytest/pull/13928/files#r2514197778

(cherry picked from commit bc0db40)

Author: webknjaz

tox.ini

@@ -23,6 +23,26 @@ envlist =
 
 
 
+[pkgenv]
+# NOTE: This section tweaks how Tox manages the PEP 517 build
+# NOTE: environment where it assembles wheels (editable and regular)
+# NOTE: for further installing them into regular testenvs.
+#
+# NOTE: `[testenv:.pkg]` does not work due to a regression in tox v4.14.1
+# NOTE: so `[pkgenv]` is being used in place of it.
+# Refs:
+# * https://github.com/tox-dev/tox/pull/3237
+# * https://github.com/tox-dev/tox/issues/3238
+# * https://github.com/tox-dev/tox/issues/3292
+# * https://hynek.me/articles/turbo-charge-tox/
+#
+# NOTE: The `SETUPTOOLS_SCM_PRETEND_VERSION_FOR_PYTEST` environment
+# NOTE: variable allows enforcing a pre-determined version for use in
+# NOTE: the wheel being installed into usual testenvs.
+pass_env =
+  SETUPTOOLS_SCM_PRETEND_VERSION_FOR_PYTEST
+
+
 [testenv]
 description =
     run the tests
@@ -49,7 +69,6 @@ passenv =
     COVERAGE_*
     PYTEST_ADDOPTS
     TERM
-    SETUPTOOLS_SCM_PRETEND_VERSION_FOR_PYTEST
     CI
 setenv =
     _PYTEST_TOX_DEFAULT_POSARGS={env:_PYTEST_TOX_POSARGS_DOCTESTING:} {env:_PYTEST_TOX_POSARGS_LSOF:} {env:_PYTEST_TOX_POSARGS_XDIST:} {env:_PYTEST_FILES:}
@@ -141,8 +160,6 @@ setenv =
 description =
     regenerate documentation examples under `{basepython}`
 changedir = doc/en
-passenv =
-    SETUPTOOLS_SCM_PRETEND_VERSION_FOR_PYTEST
 deps =
     PyYAML
     regendoc>=0.8.1