add black and flake8 to requirements-dev.txt

[dlm6693]

Description of changes: Pins versions of black and flake8 in requirements-dev.txt.

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[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (91b17f1) 88.86% compared to head (ffd6db5) 88.86%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #95 +/- ## ======================================== Coverage 88.86% 88.86% ======================================== Files 34 34 Lines 1967 1967 ======================================== Hits 1748 1748 Misses 219 219 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `88.86% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=awslabs#carryforward-flags-in-the-pull-request-comment) to find out more.

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