Add type checking for hook specifications

I think it would be helpful to support type annotations in hook specifications.

It isn't hard to add the necessary annotations to a hook specification but I couldn't work out how to integrate this with pluggy. I spent some time on this and worked out the specifics:

1. pluggy.HookspecMarker must be modified with a type hint so that the decorator does not obscure the type hints added to the specification.
2. When a hook is registered the .hook attribute of the pluggy.manager.PluginManager instance myst be cast so that mypy can connect the specification to the registered hooks.

Here is a full example:

import pluggy  # type: ignore
from typing import TypeVar, Callable, Any, cast

# Improvement suggested by @oremanj on python/typing gitter
F = TypeVar("F", bound=Callable[..., Any])
hookspec = cast(Callable[[F], F], pluggy.HookspecMarker("myproject"))
hookimpl = pluggy.HookimplMarker("myproject")


class MySpec(object):
    """A hook specification namespace."""

    @hookspec
    def myhook(self, arg1: int, arg2: int) -> int:
        """My special little hook that you can customize."""


class Plugin_1(object):
    """A hook implementation namespace."""

    @hookimpl
    def myhook(self, arg1: int, arg2: int) -> int:
        print("inside Plugin_1.myhook()")
        return arg1 + arg2 + 'a'


# create a manager and add the spec
pm = pluggy.PluginManager("myproject")
pm.add_hookspecs(MySpec)

# register plugins
pm.register(Plugin_1())

# Add cast so that mypy knows that pm.hook
# is actually a MySpec instance. Without this
# hint there really is no way for mypy to know
# this.
pm.hook = cast(MySpec, pm.hook)

# Uncomment these when running through mypy to see
# how mypy regards the type
# reveal_type(pm.hook)
# reveal_type(pm.hook.myhook)
# reveal_type(MySpec.myhook)

# this will now be caught by mypy
results = pm.hook.myhook(arg1=1, arg2="1")
print(results)

Output when checking with mypy:

$ mypy plug.py
plug.py:24: error: Unsupported operand types for + ("int" and "str")
plug.py:47: error: Argument "arg2" to "myhook" of "MySpec" has incompatible type "str"; expec
ted "int"

My original StackOverflow question and answer: https://stackoverflow.com/questions/54674679/how-can-i-annotate-types-for-a-pluggy-hook-specification

Start by inspecting pluggy.HookspecMarker and PluginManager, especially hook registration and the pm.hook attribute. Use the issue's MySpec example and mypy checks to understand the desired annotation behavior. Done should provide type checking for hook specifications without requiring the demonstrated manual casts, with coverage for the relevant APIs.