Kick off Hinty (#2158):

• turn on Mypy strict mode (see this)
• add typings to
  • packet.py
  • fields.py
  • config.py
  • main.py (strict)
  • __init__.py (strict)

Implementations notes:

• this tries to be as clean and extensible as possible: we use Generic classes to make clean Fields. This means any other field that inherits from Field will be able to mark what Internal and Machine types it uses.
• this PR fixes a few type errors that mypy triggered. For instance a function that would crash in SndRcvList and was moved to a parent class.
• I refactored TimeSignedField because it was easier than cleaning it up.
• This is fully compatible except one single thing: the use of Generic makes it impossible to use the keyword cls= as a key parameter. This keyword was only used in PacketListField (in other places, it was only used as an argument which is fine). Therefore I had to rename it to pkt_cls, but it's use was not that widespread because you would typically use it as an argument instead of a key argument.

To reproduce the issue with Generic:

from typing import Generic, TypeVar
I = TypeVar("I")
class A(Generic[I]):
    def __init__(self, cls=1):
        pass

A(cls=1)  # Error

• This is a big PR, but had to be done in one go because those files just keep referring to each other. Making them one by one would mean adding a gazillion temporary # type: ignore. I hesitated to also do utils.py, but it's honnestly to big on its own and will need a separate PR.
• python 2 compatibility was added, but it's quite a nightmare. See the code. I sometimes wish we had drop it. In any case, when will want to drop it (likely in a few years), we'll have the great com2ann module to do it for us.