Fix #12290: crash_log_parser on py3

[mark-edgeworth]

Summary of changes

Fixes a stack trace when calling crash_log_parser.py under Python 3

See #12290

Impact of changes

Migration actions required

Documentation

See See #12290 for a problem summary.

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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

Hand testing only using an existing installation. I do not have all of the relevant files to make this utility work but I can provoke the problem as reported and the changes here do fix that.
[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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[mark-edgeworth]

I do not have all of the relevant files to make this utility work but I can provoke the problem as reported and the changes here do fix that.

[0xc0170]

CI started

[mbed-ci]

Test run: FAILED

Summary: 1 of 11 test jobs failed
Build number : 1
Build artifacts

Failed test jobs:

• jenkins-ci/mbed-os-ci_dynamic-memory-usage