`con` argument of `pandas.to_sql`
```python
import pandas as pd
import pymysql


conn = pymysql.connect(user='root', password='123456', db='test', charset='utf8')
df1 = pd.read_sql('select * from videos limit 100', conn)
df1.to_sql('video2', conn)

```
as you can see above, with a pymysql connection, `pd.read_sql` works fine, but `pd.to_sql` will raise an Error:
```python
DatabaseError: Execution failed on sql 'SELECT name FROM sqlite_master WHERE type='table' AND name=?;': not all arguments converted during string formatting
```
i have searched for reason, it looks like the `con` argument of `pandas.to_sql` must be sqlalchemy.engine.Engine or sqlite3.Connection,. why doesn't it support pymysql.connection as `pd.read_sql` do?
