async branch: with statement micro benchmark

t.py

class CM:
    def __enter__(self):
        global CNT1
        CNT1 +=1

    def __exit__(self, et, e, tv):
        global CNT2
        CNT2 += 1


import time

N = 10**6

for _ in range(5):
    CNT1 = CNT2 = CNT3 = 0

    started = time.time()
    for _ in range(N):
        with CM():
            CNT3 += 1

    print('%.3fs' % (time.time() - started,))
    assert CNT1 == CNT2 == CNT3 == N

"""

yury@ysmbp ~/dev/py/cpython-git (await *$%) $ ./python.exe t.py
0.727s
0.734s
0.730s
0.718s
0.717s
yury@ysmbp ~/dev/py/cpython-git (await *$%) $ ./python.exe t.py
0.719s
0.711s
0.710s
0.748s
0.805s


yury@ysmbp ~/dev/py/cpython-git (master $%) $ ./python.exe t.py
0.722s
0.719s
0.722s
0.732s
0.737s
yury@ysmbp ~/dev/py/cpython-git (master $%) $ ./python.exe t.py
0.721s
0.701s
0.698s
0.710s
0.714s

"""