Cannot install opencv-python for PyPy2.7 - python-2.7

Question

I installed PyPy2.7 in the WSL (Ubuntu 18.04) following the official instructions. I then created a virtualenv and installed Numpy and Scipy with pip install. So far everything works:
$ python
Python 2.7.18 (63df5ef41012, Apr 04 2021, 10:19:14)
[PyPy 7.3.4 with GCC 7.3.1 20180303 (Red Hat 7.3.1-5)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>> import numpy
>>>> import scipy
>>>>
Now I want to install opencv-python, which results in the error described here. This is to be expected. However the suggested solution, running pip install opencv-python==4.2.0.32, doesn't work either
ERROR: Could not find a version that satisfies the requirement opencv-python==4.2.0.32 (from versions: 3.4.10.37, 4.3.0.38)
ERROR: No matching distribution found for opencv-python==4.2.0.32
Somehow only two versions are available and 4.2.0.32 is not one of them. Running pip install opencv-python==3.4.10.37 (which should be old enough to support python2) again throws the original error
Collecting opencv-python==3.4.10.37
Using cached opencv-python-3.4.10.37.tar.gz (87.5 MB)
Installing build dependencies ... done
Getting requirements to build wheel ... error
ERROR: Command errored out with exit status 1:
command: /mnt/d/portProg/pbMoMa-master/my-pypy/bin/pypy /mnt/d/portProg/pbMoMa-master/my-pypy/site-packages/pip/_vendor/pep517/_in_process.py get_requires_for_build_wheel /tmp/tmpNuRIPK
cwd: /tmp/pip-install-nj4gRx/opencv-python
Complete output (22 lines):
Traceback (most recent call last):
File "/mnt/d/portProg/pbMoMa-master/my-pypy/site-packages/pip/_vendor/pep517/_in_process.py", line 280, in <module>
main()
File "/mnt/d/portProg/pbMoMa-master/my-pypy/site-packages/pip/_vendor/pep517/_in_process.py", line 263, in main
json_out['return_val'] = hook(**hook_input['kwargs'])
File "/mnt/d/portProg/pbMoMa-master/my-pypy/site-packages/pip/_vendor/pep517/_in_process.py", line 114, in get_requires_for_build_wheel
return hook(config_settings)
File "/tmp/pip-build-env-tZXZdx/overlay/site-packages/setuptools/build_meta.py", line 146, in get_requires_for_build_wheel
return self._get_build_requires(config_settings, requirements=['wheel'])
File "/tmp/pip-build-env-tZXZdx/overlay/site-packages/setuptools/build_meta.py", line 127, in _get_build_requires
self.run_setup()
File "/tmp/pip-build-env-tZXZdx/overlay/site-packages/setuptools/build_meta.py", line 243, in run_setup
self).run_setup(setup_script=setup_script)
File "/tmp/pip-build-env-tZXZdx/overlay/site-packages/setuptools/build_meta.py", line 142, in run_setup
exec(compile(code, __file__, 'exec'), locals())
File "setup.py", line 448, in <module>
main()
File "setup.py", line 99, in main
% {"ext": re.escape(sysconfig.get_config_var("EXT_SUFFIX"))}
File "/mnt/d/portProg/pbMoMa-master/my-pypy/lib-python/2.7/re.py", line 210, in escape
s = list(pattern)
TypeError: 'NoneType' object is not iterable
----------------------------------------
ERROR: Command errored out with exit status 1: /mnt/d/portProg/pbMoMa-master/my-pypy/bin/pypy /mnt/d/portProg/pbMoMa-master/my-pypy/site-packages/pip/_vendor/pep517/_in_process.py get_requires_for_build_wheel /tmp/tmpNuRIPK Check the logs for full command output.