fix the python 3.7 compatibility

[albertwh1te]

Fixes issue #317

[codecov]

Codecov Report

Merging #357 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #357   +/-   ##
=======================================
  Coverage   93.17%   93.17%           
=======================================
  Files           9        9           
  Lines        1129     1129           
  Branches      161      161           
=======================================
  Hits         1052     1052           
  Misses         54       54           
  Partials       23       23

Impacted Files	Coverage Δ
aiomysql/cursors.py	94.06% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 298d2f1...f260d5b. Read the comment docs.

[terricain]

Looks good. 👍

[anyUesr]

0.0.20 do not fix this, please recheck it:

In [15]: import aiomysql                                                                                                                                                            

In [16]: aiomysql.__version__                                                                                                                                                       
Out[16]: '0.0.20'

In [17]: async def test(): 
    ...:     async with mysql_pool.acquire() as conn: 
    ...:         async for one_feature_info_res in conn.execute(FeatureInfoT.select()): 
    ...:             print(one_feature_info_res) 
    ...:                                                                                                                                                                            

In [18]: loop.run_until_complete(test())                                                                                                                                            
/home/redis/.pyenv/versions/3.7.2/bin/ipython:3: RuntimeWarning: coroutine '_SAConnectionContextManager.__aiter__' was never awaited
  # -*- coding: utf-8 -*-
RuntimeWarning: Enable tracemalloc to get the object allocation traceback
/home/redis/.pyenv/versions/3.7.2/bin/ipython:3: RuntimeWarning: coroutine 'SAConnection._execute' was never awaited
  # -*- coding: utf-8 -*-
RuntimeWarning: Enable tracemalloc to get the object allocation traceback
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-18-66585a8d0722> in <module>
----> 1 loop.run_until_complete(test())

~/.pyenv/versions/3.7.2/lib/python3.7/asyncio/base_events.py in run_until_complete(self, future)
    582             raise RuntimeError('Event loop stopped before Future completed.')
    583 
--> 584         return future.result()
    585 
    586     def stop(self):

<ipython-input-17-fa7af2a10cfe> in test()
      1 async def test():
      2     async with mysql_pool.acquire() as conn:
----> 3         async for one_feature_info_res in conn.execute(FeatureInfoT.select()):
      4             print(one_feature_info_res)
      5 

TypeError: 'async for' received an object from __aiter__ that does not implement __anext__: coroutine