inspect: Implement a very basic signature function.

[dpgeorge]

This implements a very basic inspect.signature() function.

At the moment it returns only a simple Signature instance with a parameters attribute that holds an OrderedDict whose length matches the arity of the input function (the number of arguments it takes).

So, the following code works and is compatible with CPython:

def f(a, b, *, c):
    pass

print(len(inspect.signature(f).parameters))

That should print 3.

I tested this on unix 64-bit, unix nanbox (32-bit), webassembly (32-bit) and a 32-bit bare-metal target.

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The main aim here is to support PyScript which is currently using a very unreliable hack to work out if a Python function takes arguments or not: https://github.com/pyscript/pyscript/blob/main/core/src/stdlib/pyscript/events.py#L96-L126

[dpgeorge]

@WebReflection @ntoll this will allow you to use inspect.signature() in your "when" event callback code.

[ntoll]

Bravo @dpgeorge 🎉 - this is really helpful for us (and removes another "messy" conditional around the type of interpreter in our code). 🏆