Added a DynamoDb backend

[ajenkinski]

Adds an AWS DynamoDB cache backend to contrib. The DynamoDbCache class has documentation on usage.

• Fixes #333

Checklist:

• [ ] Add tests that demonstrate the correct behavior of the change. Not applicable. I saw a rejected PR for another backend where you said you didn't want tests for contributed backends. I can add tests if you like.
• [x] Add or update relevant docs, in the docs folder and in code.
• [x] Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• [x] Add .. versionchanged:: entries in any relevant code docs.
• [ ] Run pre-commit hooks and fix any issues. All the pre-commit steps complete except the "black" stage. See below for details.
• [x] Run pytest and tox, no tests failed.

When the pre-commit hook runs, this is the output I get:

pyupgrade................................................................Passed
Reorder python imports...................................................Passed
black....................................................................Failed
- hook id: black
- exit code: 1

Traceback (most recent call last):
  File "/root/.cache/pre-commit/repotslw8qcx/py_env-python3.7/bin/black", line 8, in <module>
    sys.exit(patched_main())
  File "/root/.cache/pre-commit/repotslw8qcx/py_env-python3.7/lib/python3.7/site-packages/black/__init__.py", line 1282, in patched_main
    patch_click()
  File "/root/.cache/pre-commit/repotslw8qcx/py_env-python3.7/lib/python3.7/site-packages/black/__init__.py", line 1268, in patch_click
    from click import _unicodefun  # type: ignore
ImportError: cannot import name '_unicodefun' from 'click' (/root/.cache/pre-commit/repotslw8qcx/py_env-python3.7/lib/python3.7/site-packages/click/__init__.py)

flake8...................................................................Passed
fix UTF-8 byte order marker..............................................Passed
Trim Trailing Whitespace.................................................Passed
Fix End of Files.........................................................Passed

It doesn't seem to have anything to do with my code, and I couldn't figure out what's installing stuff in that ~/.cache directory. It seems that the code in black/__init__.py expects click to have a _unicodefun symbol, which isn't present in the version of click that's getting installed in that .cache directory. Since I didn't change anything in the requirements files, this doesn't seem related to my changes.

[northernSage]

Hey @ajenkinski, thanks for the contribution! This look good to me, but with #308 we are in the process of moving all backend cache classes to cachelib so this should probably be moved there. The pre-commit problem seems to have been fixed with #335 (new black release), so it should be fine now.

[ajenkinski]

Would you like me to move it to cachelib, or will you? If I do it, where should it go? I don't see a contrib package in cachelib.

[northernSage]

@ajenkinski That would be very helpful if you find the time. All cache classes in cachelib have their own separate modules which are all located in https://github.com/pallets-eco/cachelib/tree/main/src/cachelib. You would have to create a new one for your new DynamoDbCache (something like dynamo.py) and add to https://github.com/pallets-eco/cachelib/blob/main/src/cachelib/init.py like the other ones. Feel free to ping ('@') me if you have any questions. Thanks!