Use Python 2 compatible configparser explicitly

[Panfilwk]

What does this PR do?

Fixes install errors on python 2 with pip<9
Fixes #584

Description of the Change

Specifies the dependency on configparser to explicitly be configparser<5

Alternate Designs

Ideally this restriction would only be imposed when necessary (pip<9 is being used), but I don't believe there's a way to dynamically alter package requirements based on the version of pip being used.

Possible Drawbacks

Missing new features backported through configparser if they ever reintroduce python 2 support in versions >= 5

Verification Process

Built a new python 2 wheel with python setup.py sdist bdist_wheel
Created a virtual envrionment with python 2.7.12 and pip 8.1.1
Installed the newly created wheel file into that virtualenvironment using that older version of pip

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.

[zippolyte]

/azp run DataDog.datadogpy.integration

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[zippolyte]

LGTM, thanks for the fix 🙇‍♂️