fix(client/linters/mypy): call mypy incorrectly

[yxliang01]

It's supposed to call mypy in the root package directory with the relative path to the python file being checked. This commit enforces this behavior.

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Has a news entry file (remember to thank yourself!)
• Appropriate comments and documentation strings in the code
• Has sufficient logging.
• Has telemetry for enhancements.
• Unit tests & system/integration tests are added/updated
• Test plan is updated as appropriate
• package-lock.json has been regenerated by running npm install (if dependencies have changed)
• The wiki is updated with any design decisions/details.

Before this PR is merged, you may use https://gist.github.com/yxliang01/b975fffa0b145d75ed1dba5b3a9ad960 as the mypy wrapper to achieve the same effect.

Thanks @harpaj for the idea.
Related to #5326

[yxliang01]

I couldn't really think of any suitable test case for this. Any suggestion? Or, can I just mark test case check as checked?

[DonJayamanne]

Apologies for never getting back to you on this issue. Somehow we missed this.
I can add the necessary tests.
And as always, thank you very much for the fix.

[DonJayamanne]

@yxliang01 I'll add the tests for the PR.
Let me try to merge this and check (closing to restart CI)

[DonJayamanne]

@yxliang01
Thanks for this PR. I'd like to merge this in, however the tests are failing, please could you fix the failing tests.
I can add new tests for the code you have written or I can share them with you (I'm unable to push my changes up to your branch).

Here are the tests (You can add them Or i can after I merge your PR) DonJayamanne@9225b6e

[yxliang01]

@DonJayamanne Hmm wondering how this going?

[DonJayamanne]

@karthiknadig /cc

[msftclas]

All CLA requirements met.

[karthiknadig]

@yxliang01 I added the tests and news item. You have to sign the CLA before i can merge.

[yxliang01]

@DonJayamanne @karthiknadig CLA signed :)