fix: support executemany

[zeromake]

run example

error: aiomysql doesn't support executemany

import asyncio
import sqlalchemy as sa
from sqlalchemy.sql.ddl import CreateTable, DropTable
from aiomysql.sa import create_engine
from sqlalchemy.sql.expression import bindparam

metadata = sa.MetaData()
User = sa.Table(
    'user',
    metadata,
    sa.Column(
        'id',
        sa.Integer,
        autoincrement=True,
        primary_key=True,
        nullable=False,
    ),
    sa.Column(
        'name',
        sa.String(16),
        nullable=False,
    )
)

async def test_executemany(loop):
    engine = await create_engine(
         user="aiomysql",
         db="test_pymysql",
         host="127.0.0.1",
         password="mypass",
         port=3306,
         loop=loop,
         echo=True,
    )
    async with engine.acquire() as conn:
        await conn.execute(CreateTable(User))
        await conn.execute(User.insert().values([{"name": "test1"}, {"name": "test2"}]))
        async with conn.execute(User.select()) as cursor:
            user = await cursor.fetchone()
            assert user.id == 1
            assert user.name == "test1"
            user = await cursor.fetchone()
            assert user.id == 2
            assert user.name == "test2"
        sql = User.update().where(User.c.id == bindparam("id")).values({"name": bindparam("name")})
        await conn.execute(sql, [{"id": 1, "name": "t1"}, {"id": 2, "name": "t2"}])
        async with conn.execute(User.select()) as cursor:
            user = await cursor.fetchone()
            assert user.id == 1
            assert user.name == "t1"
            user = await cursor.fetchone()
            assert user.id == 2
            assert user.name == "t2"


loop = asyncio.get_event_loop()
loop.run_until_complete(test_executemany(loop))

[codecov]

Codecov Report

Merging #324 into master will increase coverage by 0.11%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master     #324      +/-   ##
==========================================
+ Coverage   92.91%   93.02%   +0.11%     
==========================================
  Files           9        9              
  Lines        1129     1147      +18     
  Branches      161      164       +3     
==========================================
+ Hits         1049     1067      +18     
  Misses         56       56              
  Partials       24       24

Impacted Files	Coverage Δ
aiomysql/sa/connection.py	93.26% <93.75%> (+0.69%)	⬆️

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[terricain]

@jettify your knowledge of sqlalchemy exceeds mine so you probably best suited to review this. I had a look over the diff and it looks sane though.

[ZmingZinnia]

same requirements

[jettify]

thanks a lot!