Support mkl-service in virtual environment out of the box

[ekomarova]

Since location of "Library\bin" in the virtual environment is not on the default search path, importing of mkl_service fails.
This change introduces "_init_helper.py" file which implements the following logic using built-in os Python module:

• If os.add_dll_directory attribute exists, and VIRTUAL_ENV environment variable is set, and os.path.join(os.environ["VIRTUAL_ENV"], "Library", "bin") folder exists, call os.add_dll_directory with that directory

[urob]

Why not dynamically detect the location of the libraries using metadata? This works more general as long as the mkl package is installed in the runtime environment. In particular, it would would also take care of #23.

from importlib.metadata import files

dll = next(p for p in files("mkl") if p.match("*mkl_rt*.dll"))
dll_dir = dll.locate().resolve().parent
os.add_dll_directory(dll_dir)

Here's a complete patch which handles errors and also works on Linux:
https://github.com/urob/numpy-mkl/blob/main/patches/mkl-service/dll_directory.patch

If this looks good, I'd be happy to submit a PR along those lines.

[ekomarova]

Can I merge it? Any objections?