This continues what started being discussed in #1405. The Pin API as far as the constructor (and the init()) method is concerned needs to be modified a bit in order to make it more general and suitable for a wide range of MCUs. Here's my proposal:

Pin(pin, mode, *, type=Pin.PP, pull=None, af=0)

• mode can be IN, OUT or AF
• type can be PP or OD. Irrelevant when mode=Pin.IN.
• pull can be PULL_UP, PULL_DOWN or None (or PULL_NONE but we could save on QSTRs)
• af is only relevant when mode=Pin.AF. Alternate functions can vary widely from micro to micro so I think it's OK for the values of this param to be port specific.