Deprecate pygame.image.tostring and fromstring

[SSS-Says-Snek]

Fixes #2085

Since to/fromstring's gonna be deprecated, I also replaced usage of them in tests to to/frombytes
Finally, like I mentioned on the discord, because to/frombytes and to/fromstring's docs were the exact same and made it hard to follow, I replaced it with a redirection to to/frombytes

[Starbuck5]

Thanks for working on this!

My only review notes is for the docs:

• The functions with removed documentation should at least say something like "This is the same functionality as frombytes"
• The reference to the new function should be a sphinx link to the new function. For example, ":func:`pygame.image.load()`" is a link to the function.

In case you haven't built the docs before, do python setup.py docs (with sphinx installed), and then access them with python -m docs

[SSS-Says-Snek]

Alright, I'll fix it when I get back

[SSS-Says-Snek]

Soo I won't be back for 6 more days💀

[novialriptide]

🚗

[Starbuck5]

Looks good to me!

[ankith26]

LGTM, thanks for the PR! 🎉

Left a minor nitpick-ey review, resolve at will