bug: Support PEP-517 in 6.x

[mbeacom]

Support PEP-517 in 6.x

The 6.x release series presently doesn't support PEP-517 build systems (so things like poetry totally fail to install if poetry add elasticsearch-curator@^6 for example.

Expected Behavior

Running pip wheel --no-cache-dir --use-pep517 "elasticsearch-curator (==6.0.0)" should succeed.

Actual Behavior

Fails due to pyyaml==6.0.0 as opposed to pyyaml==6.0.1.

Steps to Reproduce the Problem

1. pip wheel --no-cache-dir --use-pep517 "elasticsearch-curator (==6.0.0)"
2. poetry add elasticsearch-curator@^6

Specifications

• Version: 6.0.0
• Platform: Linux/MacOS/Windows/Anything
• Subsystem: N/A

Context (Environment)

We are presently instrumenting a lambda function to handle curation for a 6.x ES cluster. This isn't possible using contemporary build tools as a result of the explicit pinning to 6.0.0 over the 6.0.1 patch release addressing PEP-517.

Detailed Description

See above. I can provide further details if required, but I am raising a PR for this against 6.x.