Ensure sufficient stack size for worker threads

[ales-erjavec]

The default worker stack size for QThreadExecutor executor on macOS is insufficient to survive the default max recursion level.

I.e. The following fails with a SIGBUS error on macOS with Python 3.7.5

from qasync import QThreadExecutor

def func(a, *arg, **kwarg):
    func(a, *arg, **kwarg)

with QThreadExecutor(1) as executor:
    f = executor.submit(func, 1)
    assert isinstance(f.exception(), RecursionError)

Also it is impossible to use the official numpy wheel distributions with bundled openblas (numpy/numpy#11551) due to the default stack size.

The defaults in this PR are copied from https://github.com/python/cpython/blob/v3.9.0/Python/thread_pthread.h#L35

[codecov-io]

Codecov Report

Merging #16 (f6a4bbb) into master (fd6ae27) will decrease coverage by 0.12%.
The diff coverage is 63.63%.

@@            Coverage Diff             @@
##           master      #16      +/-   ##
==========================================
- Coverage   64.76%   64.64%   -0.13%     
==========================================
  Files           4        4              
  Lines         667      676       +9     
==========================================
+ Hits          432      437       +5     
- Misses        235      239       +4     

Impacted Files	Coverage Δ
qasync/__init__.py	74.57% <63.63%> (-0.43%)	⬇️

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[TheGreatCabbage]

Looks good, nice work!

[TheGreatCabbage]

Now published in qasync==0.11.0.