chore: cleanup linting errors and introduce strict linting checks

[ankush]

Problem: preventable errors passing through the review process. We use sider for these issues but sider is limited to "diff" while trying to be "diff-aware" it ignores the problem caused by the change in the same file.

Example:

 def (a, b):
+    return a
     return a + b

Something like this would be totally acceptable to sider, but flake8 will warn you about dead code.

Solution:

• Create a subset of the flake8 rules that MUST be cleared for all PR. This subset is critical things like "undefined variable".
• Run these in CI and provide them as pre-commit hooks too.

Additional checks:

• block committing merge conflicts
• block committing python code that's not parsable.
• Strip whitespace and EOF.
• Block committing to develop directly. (common mistake by new contributors that leads to conflicts in future)

To start using this install pre-commit using pip install pre-commit and then do pre-commit install.

TODO:

• update contribution guideline (post merge)