Update psycopg2 to 2.7.6

[pyup-bot]

This PR updates psycopg2 from 2.7.5 to 2.7.6.

Changelog

2.7.6

^^^^^^^^^^^^^^^^^^^^^^^^^^^

- Close named cursors if exist, even if `~cursor.execute()` wasn't called
(:ticket:`746`).
- Fixed building on modern FreeBSD versions with Python 3.7 (:ticket:`755`).
- Fixed hang trying to :sql:`COPY` via `~cursor.execute()` (:ticket:`781`).
- Fixed adaptation of arrays of empty arrays (:ticket:`788`).
- Fixed segfault accessing the `connection.readonly` and
`connection.deferrable` repeatedly (:ticket:`790`).
- `~psycopg2.extras.execute_values()` accepts `~psycopg2.sql.Composable`
objects (794).
- `~psycopg2.errorcodes` map updated to PostgreSQL 11.

Links

• PyPI: https://pypi.org/project/psycopg2
• Changelog: https://pyup.io/changelogs/psycopg2/
• Homepage: http://initd.org/psycopg/

[pyup-bot]

Closing this in favor of #275