AttributeError: __enter__ for presto.Connection and presto.connect

```
Python 3.7.7
PyHive 0.6.1
Hive 3.1.2
Presto 0.237
```
When connecting to Presto with `pyhive.presto` and the context manager with:

```python
from pyhive import presto

with presto.Connection(host='localhost', port=8080, database='default') as connection:
cursor = connection.cursor()
cursor.execute("SHOW TABLES")
print(cursor.fetchall())
```

I get the following error suggesting that the context manager could not enter:

```
AttributeError: __enter__
```

But with `pyhive.hive` it works:

```python
from pyhive import hive

with hive.Connection(host='localhost', port=10000, database='default') as connection:
cursor = connection.cursor()
cursor.execute("SHOW TABLES")
print(cursor.fetchall())
```

I noticed the same two behaviors with `presto.connect` and `hive.connect` respectively.