pyopencl for python 3.8 and 3.9 + bug fix

[lpagani91]

Hello,

Presently pyopencl is included in the python3.6 and python3.7 packages. Not in 3.8 and 3.9. Could it be ported ? On top of that, the py37-pyopencl package crashes with the following message :

  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/pyopencl/__init__.py", line 821, in kernel_set_scalar_arg_dtypes
    work_around_arg_count_bug=work_around_arg_count_bug)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/pyopencl/invoker.py", line 371, in generate_enqueue_and_set_args
    result = invoker_cache[cache_key]
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/pytools/persistent_dict.py", line 506, in __getitem__
    return self.fetch(key, _stacklevel=1)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/pytools/persistent_dict.py", line 600, in fetch
    hexdigest_key = self.key_builder(key)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/pytools/persistent_dict.py", line 233, in __call__
    self.rec(key_hash, key)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/pytools/persistent_dict.py", line 214, in rec
    method(inner_key_hash, key)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/pytools/persistent_dict.py", line 268, in update_for_tuple
    self.rec(key_hash, obj_i)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/pytools/persistent_dict.py", line 219, in rec
    % type(key))
TypeError: unsupported type for persistent hash keying: <class 'numpy.dtype[void]'>

the same code does not crash with py36-opencl