Import from collections.abc
Fixed additional warnings generated by Python 3.7 due to changes in the
organization of the Python ``collections`` and ``collections.abc`` packages.
Previous ``collections`` warnings were fixed in version 1.2.11. Pull request
courtesy xtreak.
See I2d1c0ef97c8ecac7af152cc56263422a40faa6bb for the original collections.abc
fixes.
Fixes: #4339
Change-Id: Ia92d2461f20309fb33ea6c6f592f7d4e7e32ae7a
Pull-request: https://github.com/zzzeek/sqlalchemy/pull/475
(cherry picked from commit
2d2fa49130249f757f6f1100dad879e1071ad385)