Update psycopg2 to 2.7.4

[pyup-bot]

There's a new version of psycopg2 available.
You are currently using 2.7.3.2. I have updated it to 2.7.4

These links might come in handy: PyPI | Changelog | Homepage

Changelog

2.7.4

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

• Moving away from installing the wheel package by default.
  Packages installed from wheel raise a warning on import. Added package
  psycopg2-binary to install from wheel instead (:ticket:543).
• Convert fields names into valid Python identifiers in
  ~psycopg2.extras.NamedTupleCursor (:ticket:211).
• Fixed Solaris 10 support (:ticket:532).
• cursor.mogrify() can be called on closed cursors (:ticket:579).
• Fixed setting session characteristics in corner cases on autocommit
  connections (:ticket:580).
• Fixed ~psycopg2.extras.MinTimeLoggingCursor on Python 3 (:ticket:609).
• Fixed parsing of array of points as floats (:ticket:613).
• Fixed ~psycopg2.__libpq_version__ building with libpq >= 10.1
  (:ticket:632).
• Fixed ~cursor.rowcount after ~cursor.executemany() with :sql:RETURNING
  statements (:ticket:633).
• Fixed compatibility problem with pypy3 (:ticket:649).
• Wheel packages compiled against PostgreSQL 10.1 libpq and OpenSSL 1.0.2n.
• Wheel packages for Python 2.6 no more available (support dropped from
  wheel building infrastructure).

Got merge conflicts? Close this PR and delete the branch. I'll create a new PR for you.

Happy merging! 🤖