[Internal] Remove unused decorator dependency

[TheKevJames]

What does this PR do?

This PR removes the unused dependency on decorator, which as far as I can tell is completely unused. It appears this library was introduced as a dependency in the first commit to this repo but never actually used directly.

This will help make installations faster and slimmer as well as reducing some maintenance (eg. see #646, where the datadogpy build was broken due to environment mismatches in this dependency).

Description of the Change

Removes the unused dependency from the setup.py file. This will prevent it from being included at installation time of datadogpy.

Alternate Designs

N/A

Possible Drawbacks

If the decorator library is being used in some non-standard way, this could remove something we're relying on (note: see "Verification Process" for why I don't think this is the case).

Verification Process

I verified this change through:

• manual audit of datadogpy: no instances of accessing the decorator library were found
• running the tox unit and admin-only tests on all compatible cpython and pypy versions: all passed
• auditing the decorator codebase for anything "unusual" such as install-time side-effects or other such things we might be "magically" relying on: did not find any

Additional Notes

N/A

Release Notes

N/A

Review checklist (to be filled by reviewers)

• Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have one changelog/ label attached. If applicable it should have the backward-incompatible label attached.
• PR should not have do-not-merge/ label attached.
• If Applicable, issue must have kind/ and severity/ labels attached at least.

[TheKevJames]

The contributing instructions tell me to add a changelog label (probably changelog/no-changelog in this case?) but I do not seem to have permissions to do so. Is there a process I am unaware of here?

[therve]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[therve]

Thanks for your patch!