Update python versions used for testing in main (facebookresearch#2679)

Summary:
New version of fairscale breaks with python3.6. Moving to higher versions of python.

# Before submitting

- [ ] Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
- [ ] Did you read the [contributor guideline](https://github.com/pytorch/fairseq/blob/main/CONTRIBUTING.md)?
- [ ] Did you make sure to update the docs?
- [ ] Did you write any new necessary tests?

## What does this PR do?
Fixes # (issue).

## PR review
Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

## Did you have fun?
Make sure you had fun coding �

Pull Request resolved: fairinternal/fairseq-py#2679

Reviewed By: alexeib

Differential Revision: D32653241

Pulled By: dianaml0

fbshipit-source-id: bb81023b967f7d585fe9fb808f65cfd60cf51490

.github/workflows/build.yml

@@ -14,7 +14,7 @@ jobs:
       max-parallel: 4
       matrix:
         platform: [ubuntu-latest, macos-latest]
-        python-version: [3.6, 3.7]
+        python-version: [3.8, 3.9]
 
     runs-on: ${{ matrix.platform }}