MAINT: bump minimum packaging version to 23.2

pyproject-metadata 0.9.1 requires packaging 23.2 or later, but does not record
this dependency, see pypa/pyproject-metadata#239

This becomes a problem when meson-python is installed on a platform where
packaging 19.0 (the current minimum required version) is installed. This does
not trigger an update of packaging to the latest available version and results
in run time errors, see scipy/scipy#22840 (comment)

This could be fixed by a new release of pyproject-metadata, but we do not want
meson-python to depend on very recent releases. meson-python supports
pyproject-metadata 0.9.0 or later and still carries some work-arounds for
issues in that release.

The least invasive and most effective solution is to bump the packaging
requirement for meson-python to 23.2.

This also allows to remove a more restrictive packaging version requirement
used for the tests optional dependencies, due to the use of API available only
with packaging 23.1 or later in the meson-python tests.

Fixes #733.

Author: dnicolodi

pyproject.toml

@@ -8,7 +8,7 @@ backend-path = ['.']
 requires = [
   'meson >= 0.64.0; python_version < "3.12"',
   'meson >= 1.2.3; python_version >= "3.12"',
-  'packaging >= 19.0',
+  'packaging >= 23.2',
   'pyproject-metadata >= 0.9.0',
   'tomli >= 1.0.0; python_version < "3.11"',
 ]
@@ -36,15 +36,14 @@ classifiers = [
 dependencies = [
   'meson >= 0.64.0; python_version < "3.12"',
   'meson >= 1.2.3; python_version >= "3.12"',
-  'packaging >= 19.0',
+  'packaging >= 23.2',
   'pyproject-metadata >= 0.9.0',
   'tomli >= 1.0.0; python_version < "3.11"',
 ]
 
 [project.optional-dependencies]
 test = [
   'build',
-  'packaging >= 23.1',
   'pytest >= 6.0',
   'pytest-cov[toml]',
   'pytest-mock',