Improve typing for function uuid7

[mahmoudajawad]

This change adds overload variants for uuid7 function to vet the return type based on value of argument as_type.

Unfortunately, this was made possible only by making arguments as_type, time_func, _last, and _last_as_of keyword-only. However, I assume this should be fine as:

• This lib has not been published as stable v1 API.
• The provided example in function documentation uses keyword when referred to as_type.
• Other arguments are barely to be used in most of the cases for generating UUIDs.

With this change:

uuid7() # revealed type is: UUID
uuid7(as_type=None) # revealed type is: UUID
uuid7(as_type="str") # revealed type is: str
uuid7(as_type="int") # revealed type is: int
uuid7(as_type="bytes") # revealed type is: bytes

This change takes in mind that current function documentation referred to use of argument ns as positional argument, and as such, argument ns was exempted from being also keyword-only.

Looking forward to hearing any feedback on this, or seeing it merged if the breaking changes are within the acceptable range.