Update PyPI classifiers to include py38

[seanpmorgan]

Description of changes:
Include py38 in PyPI classifier. Keeps it in parity with sagemaker SDK. Similar PR to aws/sagemaker-python-sdk#1626

This PR will also update the badge on the central README after the next release.

Testing done:
Minimal tests ran on py38 environment. Shouldn't be any incompatibilities that I can think of.

Merge Checklist

General

• I have read the CONTRIBUTING doc
• I used the commit message format described in CONTRIBUTING
• I have used the regional endpoint when creating S3 and/or STS clients (if appropriate)
• I have updated any necessary documentation, including READMEs

Tests

• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have checked that my tests are not configured for a specific region or account (if appropriate)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[seanpmorgan]

Also, why do we test this on SageMaker SDK <v2.x?

[sagemaker-bot]

AWS CodeBuild CI Report

• CodeBuild project: sagemaker-training-toolkit-pr
• Commit ID: 2fab4f3
• Result: SUCCEEDED
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[satishpasumarthi]

SageMaker Training toolkit already supports py38.