Systematically replace __del__ with weakref.finalize()

[rwgk]

Closes #141

See #141 (comment) for the rationale.

Note that Event and Stream are a bit special, because the regular __init__()s raise. The pattern adopted for both is to add _enable_finalize(), which NEEDS to be called from derived classes; that is a potential trap. Should we try to avoid that trap?

[leofang]

Thanks, Ralf! A general comment: We can't use self.close as the finalizer because it'd still hold a reference to self (see @wence-'s original #87 (comment)). The most simple thing we could do is to move it out of the class, so instead of

class Something:

    def __init__(self, ...):
        self._handle = ...
        weakref.finalize(self, self.close)

    def close(self, ...):
        if self._handle:
            # do something

we do

# note the 1st arg name is still "self", to mimic a method
def _close_something(self, ...):
    if self._handle:
        # do something

class Something:

    def __init__(self, ...):
        self._handle = ...
        weakref.finalize(self, _close_something)

[rwgk]

Ah, confirmed, by looking more. Yesterday I was jumping to a wrong conclusion, twisty explanation omitted. I'll try again.

[rwgk]

I think I found a nice pattern to avoid reference cycles. This is my updated toy example:

https://github.com/rwgk/stuff/blob/7d93794069898f245e14fb94d779816291fde1ff/random_attic/weakref_finalize_toy_example.py

The trick is to introduce a level of indirection, self._members, and tie weakref.finalize() to those.

I still need to adopt that pattern in this PR.