project_wheel_metadata returns email.message.Message instead of importlib.metadata._adapters.Message in python 3.9

When using python 3.9.18 (but it happens also with other 3.9.x), project_wheel_metadata returns an instance of email.message.Message instead of importlib.metadata._adapters.Message.

The problem is that the returned object doesn't have the property json as defined by PackageMetadata

import sys
from build.util import project_wheel_metadata
metadata = project_wheel_metadata(".", isolated=False)

>>> print(sys.version)
3.10.8 (main, Nov 16 2022, 15:31:10) [Clang 14.0.0 (clang-1400.0.29.202)]
>>> print(repr(metadata))
<importlib.metadata._adapters.Message object at 0x10e8c3fa0>
>>> hasattr(metadata, "json")
True

>>> print(sys.version)
3.9.18 (main, Oct 17 2023, 16:56:25) 
[Clang 14.0.0 (clang-1400.0.29.202)]
>>> print(repr(metadata))
<email.message.Message object at 0x1015f66d0>
>>> hasattr(metadata, "json")
False

>>> print(sys.version)
3.8.15 (default, Nov 16 2022, 15:37:40) 
[Clang 14.0.0 (clang-1400.0.29.202)]
>>> print(repr(metadata))
<importlib_metadata._adapters.Message object at 0x10bb06100>
>>> hasattr(metadata, "json")
True

As it's shown above, the json property works correctly with python 3.10.8 and 3.8.15, but not with python 3.9.18.

Installed packages in python 3.10.8:

build==1.0.3
packaging==23.2
pip==23.3
pyproject_hooks==1.0.0
setuptools==68.2.2
tomli==2.0.1
wheel==0.41.2

Installed packages in python 3.9.18:

build==1.0.3
importlib-metadata==6.8.0
packaging==23.2
pip==23.3
pyproject_hooks==1.0.0
setuptools==68.2.2
tomli==2.0.1
wheel==0.41.2
zipp==3.17.0

Installed packages in python 3.8.15:

build==1.0.3
importlib-metadata==6.8.0
packaging==23.2
pip==23.3
pyproject_hooks==1.0.0
setuptools==68.2.2
tomli==2.0.1
wheel==0.41.2
zipp==3.17.0

The issue seems to be caused by the conditional import in

build/src/build/_importlib.py

Line 6 in 71bc938

    
           elif sys.version_info < (3, 9, 10) or (3, 10, 0) <= sys.version_info < (3, 10, 2):

If import importlib_metadata as metadata is executed even for python 3.9.18 (in my case), the issue seems resolved.

Thanks Gianluca for making this change. It drastically simplified the code in pytest-checkdocs.