CMake: Add requirements.txt file for required Python modules

[0xc0170]

Summary of changes

The memmap script depends on external Python modules.
The added file captures the required packages to be installed.

One question: where we capture what we use (memmap dependency is not clear) ? Shall I add a note about additional cmake dependencies to our readme?

Impact of changes

Migration actions required

Documentation

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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

[x] No Tests required for this change (E.g docs only update)
[] 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]

@0xc0170, thank you for your changes.
@ARMmbed/mbed-os-tools @ARMmbed/mbed-os-maintainers please review.

[0xc0170]

@rajkan01 Please review

[mergify]

This PR cannot be merged due to conflicts. Please rebase to resolve them.

Pull request has been modified.

[0xc0170]

CI started

[mbed-ci]

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jenkins-ci/mbed-os-ci_build-example-ARM	✔️
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