This is mostly similar to i16 that I added recently in #15464, but there are some
differences:

• Some adjustments were needed to support unsigned integers
• Add overflow checking of literals, since it's easy to over/underflow when using u8 due to limited range
• Rename primitive integer types from int16 to i16 (etc.) to match the user-visible types (needed to get some error messages consistent, and it's generally nicer)
• Overall make things a bit more consistent
• Actually update mypy_extensions stubs

This is an unsigned type to make it easier to work with binary/bytes data. The item values for bytes are unsigned 8-bit values, in particular. This type will become much more useful once we support packed arrays.