* convert_unicode engine option is no longer supported
* use positional arguments for select, lists are no longer supported
* insert no longer accepts keyword constructor arguments
* explicitly convert Rows to dicts

Change-Id: I858d60b3d142e8ade8b2c0154872c8bfc19a42ba

This library no longer supports Python 2, thus usage of six can be
removed. This also removes workaround about pickle library used in
Python 2 only.

Change-Id: I19d298cf0f402d65f0b142dea0bf35cf992332a9

The __future__ module [1] was used in this context to ensure compatibility
between python 2 and python 3.

We previously dropped the support of python 2.7 [2] and now we only support
python 3 so we don't need to continue to use this module and the imports
listed below.

Imports commonly used and their related PEPs:
- `division` is related to PEP 238 [3]
- `print_function` is related to PEP 3105 [4]
- `unicode_literals` is related to PEP 3112 [5]
- `with_statement` is related to PEP 343 [6]
- `absolute_import` is related to PEP 328 [7]

[1] https://docs.python.org/3/library/__future__.html
[2] https://governance.openstack.org/tc/goals/selected/ussuri/drop-py27.html
[3] https://www.python.org/dev/peps/pep-0238
[4] https://www.python.org/dev/peps/pep-3105
[5] https://www.python.org/dev/peps/pep-3112
[6] https://www.python.org/dev/peps/pep-0343
[7] https://www.python.org/dev/peps/pep-0328

Change-Id: Ie7c18f2f1b6294e3d19c36112d35f4472ce77e7b

python3.0 has deprecated LOG.warn
https://docs.python.org/3/library/logging.html#logging.warning
Closes-Bug: #1650843

Change-Id: Ib8473cbdb84547f385e619fe08ed723b1ee7bcdb

This patch replaces the legacy retrying library with the newer
and more convenient tenacity one, taking into account that:
1) retrying uses milliseconds for wait times, but tenacity uses seconds;
2) retrying has a lot of numeric arguments for specifying behaviour
of decorated functions, while tenacity has a few of them, which are
specialized objects, thus making the retry-decorator more flexible.

Change-Id: I4b165d37b2ecc210f2b94c103b73eaab51529261
Closes-Bug: #1635404

Make sure that upgrade() is thread-safe (as it appears not
to be) by using a lock that all connections from the same
engine will use.

Change-Id: I2b2b1be9e797099c8412fc6819465e550b1b934a
Closes-Bug: #1559496

Instead of a custom loop and backoff just use the retrying
library to do this same code and delegate the loop complexity
to it instead.

Change-Id: Iaf02cc728d2a2cfc7077300e03d7ef25522717b7

Change-Id: I415a81d3b6b15b17a9a91cc2a0681c159172a4e1