[mbed-os-5.15] travis: fix 3.5 test bug in attrs

[artokin]

Summary of changes

pytest fetched newer attrs that caused an issue for python 3.5. We found few references with an error and they suggested to use 1.19 version.

This is copy of : #14108 to mbed-os-5.15 branch

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

[] 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]

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

[0xc0170]

Thank you ! will run CI once we get the release completed

[0xc0170]

CI started