Add support for PEP 525-style garbage collection hooks

[oremanj]

Add async_generator.get_asyncgen_hooks() and async_generator.set_asyncgen_hooks(), and use them under the same circumstances the equivalents in sys would be used for native generators. On 3.6+, the async_generator names refer to the same functions as the sys names, so there's only one set of hooks, and frameworks that use sys.set_asyncgen_hooks() should work seamlessly with async_generator functions as well.

There's an issue with GC ordering or something in pypy, which causes the new logic to make the interpreter segfault under some circumstances, so keep the old logic (which complains if any async_generator is GC'ed before its execution has completed) if we detect that we're running under pypy. The bug has been reported upstream: https://bitbucket.org/pypy/pypy/issues/2786/

[codecov[bot]]

Codecov Report

Merging #15 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff           @@
##           master    #15    +/-   ##
======================================
  Coverage     100%   100%            
======================================
  Files           7      7            
  Lines         776    972   +196     
  Branches       59     77    +18     
======================================
+ Hits          776    972   +196

Impacted Files	Coverage Δ
async_generator/__init__.py	100% <ø> (ø)	:arrow_up:
async_generator/_tests/test_async_generator.py	100% <100%> (ø)	:arrow_up:
async_generator/_impl.py	100% <100%> (ø)	:arrow_up:

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[oremanj]

ping on this?

[njsmith]

Thanks!