Run tests for python 3.7 in CI and fix failing tests

[caulagi]

Related to #357 and #317. I.e., I see the following errors with Python 3.7 -

$ py.test -s -v -x ./tests/                                                                                                    1.8m  Sun Jan 27 21:24:05 2019
Test session starts (platform: darwin, Python 3.7.2, pytest 3.9.1, pytest-sugar 0.9.1)
cachedir: .pytest_cache
rootdir: /Users/pradipcaulagi/jipu/src/aiomysql, inifile:
plugins: sugar-0.9.1, cov-2.6.0

 tests/test_async_iter.py::test_async_cursor[asyncio-5.6] ✓                                                                                                                                    0%
 tests/test_async_iter.py::test_async_cursor_server_side[asyncio-5.6] ✓                                                                                                                        0% ▏

――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――― test_async_iter_over_sa_result[asyncio-5.6] ――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――――

mysql_params = {'db': 'test_pymysql', 'host': 'localhost', 'local_infile': True, 'password': '', ...}, table = None, loop = <_UnixSelectorEventLoop running=False closed=False debug=False>

    @pytest.mark.run_loop
    async def test_async_iter_over_sa_result(mysql_params, table, loop):
        ret = []
        engine = await sa.create_engine(**mysql_params, loop=loop)
        conn = await engine.acquire()

>       async for i in (await conn.execute(tbl.select())):
E       TypeError: 'async for' received an object from __aiter__ that does not implement __anext__: coroutine

tests/test_async_iter.py:61: TypeError

The fixes are based on aiopg utils.

I added running tests for python 3.7 as part of CI.

[codecov]

Codecov Report

Merging #378 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #378      +/-   ##
==========================================
+ Coverage   92.28%   92.34%   +0.05%     
==========================================
  Files           9        9              
  Lines        1180     1189       +9     
  Branches      172      173       +1     
==========================================
+ Hits         1089     1098       +9     
  Misses         64       64              
  Partials       27       27

Impacted Files	Coverage Δ
aiomysql/sa/result.py	92% <100%> (ø)	⬆️
aiomysql/utils.py	90.09% <100%> (+0.96%)	⬆️

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[Nothing4You]

thank you for the PR.

python 3.7 support has been implemented quite a while ago, therefore this is no longer relevant.

[Nothing4You]

for the SA fix, this should be covered in #494, which also includes a test case.