[0.4.5] Soft dependencies

[retostauffer]

Testing the package on a 'naked' installation (Unix and Windows) and noticed that the one soft-dependency (suggested; matplotlib) is required for color vision deficiency simulation. To be precise:

Minimal

from colorspace import diverging_hcl, deutan
colors = diverging_hcl("Green-Orange").colors(10)
print(deutan(colors))

... where deutan() calls CVD.check_hex_colors() which imports matplotlib.colors.to_hex to allow for conversion for e.g., "red", 1.

Possible solution

I think this can be removed easily by only importing matplotlib if we do only get strings with valid hex colors by re-using the regex from colorlib.hex_to_sRGB/def: validhex() and only (try except) import matplotlib when needed.

[zeileis]

Sounds like a good idea!

But if we have to depend on matplotlib, it would be tolerable, I guess.

[retostauffer]

Fixed, make that dependent on the input and only import matplotlib when needed.

Adjusted tests and the GitHub Action workflow, now does the following:

• Runs "soft" tests: Only installs colorspace with the mandatory requirements
• Runs full tests afterwards with all packages needed for extended functionality (matplotlib, imageio, pandas).

@zeileis FYI