rez-pip can't handle python_version requirements in packages

[BryceGattis]

elastictransport 8.12.0 has the following requirements:

'urllib3 <3,>=1.26.2',
'certifi',
'dataclasses ; python_version < "3.7"',
'importlib-metadata ; python_version < "3.8"'
...
...

When running rez-pip2 elastictransport==8.12.0, I end up getting a ResourceError like this:

Traceback (most recent call last):
  File "C:\Python37\lib\runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "C:\Python37\lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "C:\Program Files\Rez\Scripts\rez-pip2.exe\__main__.py", line 7, in <module>
  File "c:\program files\rez\lib\site-packages\rez_pip\cli.py", line 324, in run
    _run(args, pipArgs, pipWorkArea)
  File "c:\program files\rez\lib\site-packages\rez_pip\cli.py", line 225, in _run
    release=args.release,
  File "c:\program files\rez\lib\site-packages\rez_pip\rez.py", line 126, in createPackage
    pkg.pip["metadata"] = remainingMetadata
  File "C:\Python37\lib\contextlib.py", line 119, in __exit__
    next(self.gen)
  File "c:\program files\rez\lib\site-packages\rez\package_maker.py", line 209, in make_package
    variant_ = variant.install(path, dry_run=True)
  File "c:\program files\rez\lib\site-packages\rez\packages.py", line 457, in install
    overrides=overrides)
  File "c:\program files\rez\lib\site-packages\rezplugins\package_repository\filesystem.py", line 951, in install_variant

    variant = _create_variant()
  File "c:\program files\rez\lib\site-packages\rezplugins\package_repository\filesystem.py", line 947, in _create_variant

    overrides=overrides
  File "c:\program files\rez\lib\site-packages\rezplugins\package_repository\filesystem.py", line 1246, in _create_variant
    % (variant, package))
rez.exceptions.ResourceError: Attempting to install a package without variants (MemoryVariantResource({'location': 'memory{0x1ea5f20c0e8}', 'name': 'elastic_transport', 'version': '8.12.0', 'index': None, 'repository_type': 'memory'})) into an existing package with variants (FileSystemPackageResource({'location': '\\\\vm-fs-003\\prod\\sww\\rez\\packages', 'name': 'elastic_transport', 'version': '8.12.0', 'repository_type': 'filesystem'}))

It seems like the python version dependent dependency was put in the variant list in the output package.py file:

variants = [['importlib_metadata']]

I am doing the rez-pip install with both Python 3.7.9 and Python 3.9.10 at the same time (by not specifying --python-version).

The issue seems to be that the Python 3.7 variant requires importlib-metadata but the 3.9 variant does not, and so the above crash happens.