Building with `pip wheel` fails from repeated patch application

[uSpike]

Expected behaviour

pip wheel path/to/opencv-python should work the same as python setup.py bdist_wheel

Actual behaviour

pip wheel https://github.com/skvark/opencv-python@34#egg=opencv-python fails because patches are applied multiple times. The patch command is run each time setup.py main() is called, and pip wheel calls setup.py multiple times.

Steps to reproduce

On a system that applies patches (currently sys.platform == 'darwin' or sys.platform.startswith('linux') and not x64), run pip wheel https://github.com/skvark/opencv-python#egg=opencv-python. The resultant output:

  ERROR: Command errored out with exit status 1:
   command: /home/me/src/piwheels/venv/bin/python -u -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-req-build-6he24q5u/setup.py'"'"'; __file__='"'"'
/tmp/pip-req-build-6he24q5u/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code,
 __file__, '"'"'exec'"'"'))' bdist_wheel -d /tmp/pip-wheel-oeddjr7s
       cwd: /tmp/pip-req-build-6he24q5u/
  Complete output (21 lines):
<snip>
    ['python/cv2[^/]*%(ext)s' % {'ext': re.escape(sysconfig.get_config_var('SO'))}],
  patching file opencv/3rdparty/openexr/IlmImf/ImfSystemSpecific.cpp
  Reversed (or previously applied) patch detected!  Assume -R? [n]
  Apply anyway? [n]
  Skipping patch.
  4 out of 4 hunks ignored -- saving rejects to file opencv/3rdparty/openexr/IlmImf/ImfSystemSpecific.cpp.rej
  Traceback (most recent call last):
    File "<string>", line 1, in <module>
    File "/tmp/pip-req-build-6he24q5u/setup.py", line 398, in <module>
      main()
    File "/tmp/pip-req-build-6he24q5u/setup.py", line 160, in main
      subprocess.check_call("patch -p0 < patches/patchOpenEXR", shell=True)
    File "/home/me/.pyenv/versions/3.8.2/lib/python3.8/subprocess.py", line 364, in check_call
      raise CalledProcessError(retcode, cmd)
  subprocess.CalledProcessError: Command 'patch -p0 < patches/patchOpenEXR' returned non-zero exit status 1.

I would suggest updating setup.py to only patch when "bdist_wheel" in sys.argv.