How do I loop through letters in Python 3?

Question

I am trying to write a function that is supposed to accept a dictionary argument to validate a Chessboard. According to the grid of a "Valid" board there are 64 possible "valid" key values that can be assigned in the dictionary.

Those are columnwise 1- 8 which I have already addressed a check for with int(key[0:-1]) > 8. How can I loop through the rows of the keys which a - h are the valid ranges?

here are some sample keys to give an example of the grid of acceptable key values: 1c, 2g, 8d, 4f, etc up to a maximum of 8h.

A friend suggested startswith(), which would be fine, but I don't know how to format that to compare the key values with startswith(). I'd like to know how I can loop through letters a, b, c, etc. As easily as you can with something like range() for numbers. Also how could I do a compare with letters like if key[-1] > 'h'? or is that even possible?

    def cBV(dic):                        # (c)hess(B)oard(V)alidator
    if not isinstance(dic, type({})):
        print('TypeError: object passed to cBV is not of type <class dict>')
        return 'VOID'

    if 'wking' and 'bking' not in dic.values():
        return False
    for key in (dic):
        try: 
            int(key[0:-1]) > 8 
            print(key)                      # to show me the "valid" key
        except ValueError:
                print(key)                  # just a personal validation check
                return False
    return True 


dict = {'8h': 'wking', '2fc': 'bking'}      # key value 2fc intentionally set to fail
test = cBV(dict)
print(test)

Here is the full code. I'm reading over that ty its a bit complex to me may take some time