Spelling.
Merge remote-tracking branch 'origin/3.0-dev' into 3.0-dev

# By Bernie Hackett (7) and Luke Lovett (1)
# Via Bernie Hackett
* origin/3.0-dev:
  Fix read preference tests.
  PYTHON-719 Read preference backward compatibility
  Make gridfs tests pass regardless of execution order.
  Fix an issue with mongos read preferences.
  PYTHON-718 - Use SSLContext when available.
  PYTHON-717 - Implement MongoCredential
  PYTHON-714 Work around localhost exception issues in add_user when connected to MongoDB >= 2.7.1.
  Fix tests under pypy3.