Python reference equality changed to value equality

[pea-pod]

Summary of changes

In tools/config/__init__.py a reference comparison (is) was used to compare a variable with a string literal, but a value comparison (==) should be used instead.

This shows up as a python warning during compilation. Python documentation is here for the obsolete Python 2.7, the minimum supported version (by the Python foundation) 3.5, and the latest 3.9.

Impact of changes

This will remove the warning emitted during compilation with the cli (and probably elsewhere).

This should fix any potential issues if cpython and friends decide to change how string caching works.

Migration actions required

None.

Documentation

None.

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[x] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

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[ciarmcom]

@pea-pod, thank you for your changes.
@ARMmbed/mbed-os-tools @ARMmbed/mbed-os-maintainers please review.

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 6 of 6 test jobs passed
Build number : 1
Build artifacts

[mergify]

This PR does not contain release version label after merging.