Add Literal type for python 3.7

[erikmannerfelt]

As of python 3.8, the Literal type hint exists by default. It is extremely useful, but cannot be used in geoutils directly as it is not supported by python 3.7! Luckily, there's a solution...

Why it's important

If I have a function that returns a different type depending on a boolean flag, types should be annotated accordingly:

def func(flag: bool = True) -> int | str:
    if flag:
         return 1
    return "one"

The problem without py38+ is that the type will be int | str no matter what flag is given.
This will always return an int if flag==True and str if not. The solution is overload and Literal:

from typing import overload, Literal

@overload
def func(flag: Literal[True])  -> int: ...

@overload
def func(flag: Literal[False])  -> str: ...

 # here goes the function definition

The solution

typing-extensions , the official backport of new typing functions!
Using conditionals in the pip and conda requirements, we can make sure that it is only installed for python <=3.7.

This has already been a big problem in #208 and #216 , so this should be fixed soon!