Raise proper TypeError in putpixel

[nulano]

Fixes #4876:

• The existing code silently ignores an OverflowError for 8bpp images, hoping that the int will become a tuple in the next few lines.
• It also fails to check that the given parameter is a tuple before trying to unpack it, giving the less than useful message SystemError: new style getargs format but argument is not a tuple.
• Last but not least, passing a non-int value for non-8bpp non-float images (e.g. I;16) gives an even less useful message SystemError: <method 'putpixel' of 'ImagingCore' objects> returned NULL without setting an error.

This PR resolves all of these issues by explicitly raising a TypeError if the parameter can't be converted to an int, or a tuple for an 8bpp image.

[hugovk]

Thanks! Do you think this should go in the release notes?

https://pillow.readthedocs.io/en/latest/releasenotes/8.0.0.html

[nulano]

Probably not. Release notes typically mention API changes or additions, and significant bugfixes. Fixed error message when an invalid color is specified does not sound very noteworthy, especially since most users will probably only encounter the error message once during development of their application.