Update lowlevel.py

[garnetri]

Adding in ctypes.util.find_library to enable cdll to find and load in the DLLs that were stored in windows environment variables. Apparently after Python 3.8 your environment variable's path is not searched for DLLs. This allows people to have the bin path added to environment variables for importing openslide without having to use the hacky os.add_dll_directory() method. Also enables sub-processes initiated by Python's multiprocessing package to properly import openslide.

[openslide-bot]

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[bgilbert]

Thanks for the PR! Loading libraries from unintended locations can have security consequences, so it's important that we use a search path that doesn't surprise users. find_library() searches for libraries the way a linker would do at compile time, not the way a runtime loader would, so it's not an appropriate function to use here. os.add_dll_directory() is the function specifically provided by Python for managing the runtime DLL search path.

If you want to fetch the OpenSlide installation directory from an environment variable, you can use something like:

with os.add_dll_directory(os.environ['OPENSLIDE_INSTALL_PATH']):
    import openslide