changed package name to flake8 to fix pytests pip install

[ChristopheBrown]

I run into this error trying to pip install for pytest pip install '.[test]':

ERROR: Could not find a version that satisfies the requirement flake==4.0.1; extra == "test" (from kubeflow-training[test]) (from versions: none)
ERROR: No matching distribution found for flake==4.0.1; extra == "test"

When changing the line to "flake8==4.0.1", I get a successful run:

Successfully built kubeflow-training
Installing collected packages: kubeflow-training
  Attempting uninstall: kubeflow-training
    Found existing installation: kubeflow-training 1.7.0
    Uninstalling kubeflow-training-1.7.0:
      Successfully uninstalled kubeflow-training-1.7.0
Successfully installed kubeflow-training-1.7.0

No changes to the version, or anything else really.

[coveralls]

Pull Request Test Coverage Report for Build 9032957488

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.05%) to 35.431%

---

Files with Coverage Reduction	New Missed Lines	%
pkg/controller.v1/mpi/mpijob_controller.go	2	81.24%

Totals
Change from base Build 9023459617:	0.05%
Covered Lines:	4380
Relevant Lines:	12362

---

💛 - Coveralls

[tenzen-y]

@ChristopheBrown Thank you for creating this PR!
Could you sign DCO in the following?

https://github.com/kubeflow/training-operator/pull/2109/checks?check_run_id=24803090584

[ChristopheBrown]

@tenzen-y Followed the steps in the page you linked. Let me know if anything else is needed. Thanks!

[tenzen-y]

@tenzen-y Followed the steps in the page you linked. Let me know if anything else is needed. Thanks!

Awsome! Thank you!
/lgtm
/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: tenzen-y

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• sdk/python/OWNERS [tenzen-y]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment