Implement context manager for Cursors

[gvenzl]

Implement context manager for Cursor object to enable with statement.
Currently the with statement can't be used for cursors because they don't implement a context manager, i.e. the __enter__ and __exit__ attributes meaning that the following code will fail:

with conn.cursor() as c:
    c.execute("SELECT 'test' from dual")
    result = c.fetchall()
    print(result)

  File “/Users/gvenzl/test/test.py”, line 23, in test
    with conn.cursor() as c:
AttributeError: __exit__

It would make sense to enable cursor object to be used in with statements that take care of closing the cursors automatically.

[anthony-tuininga]

I agree that it would make sense to implement the context manager on cursors. Thanks @gvenzl.