Make scipy and PyWavelets optional

[JanisV]

It would be great to make scipy and PyWavelets dependencies optional:
pip install imagehash[pywt] or pip install imagehash[scipy]

When scipy is missed we can use numpy implementation of 2D DCT (https://public.websites.umich.edu/~mejn/cp/programs/dcst.py). Also separating DCT function can help with #108.

If you are interested, I can create a PR

[JohannesBuchner]

What is the motivation behind this?

[JanisV]

Size of Scipy is about 166M. It's too large for my desktop app.
PyWavelets I don't need but I had a truble with it while building flatpak.

[JohannesBuchner]

Would "pip install --no-deps" be a workaround for you?
Would uninstalling scipy before packaging be a workaround for you?

I would prefer imagehash to be as useful as possible by default.

[JanisV]

Yes, no-deps or uninstalling can help with PyWavelets. But I use pHash and whant implementation without scipy

[JohannesBuchner]

Would be good to know if the libraries are bit-for-bit / bug-for-bug compatible in their output.