Extend coverage versions in requirements_dev.txt

[rkazants]

Root cause analysis: CI fails during installation of modules from requirements_dev.txt on Ubuntu 20 with python 3.8.

Solution: Extend coverage versions in requirements_dev.txt. Coverage 5.5 is the latest version for which I have successfully tested the MO unit-tests coverage.

Code:

• Comments - N/A
• Code style (PEP8) - N/A
• Transformation generates reshape-able IR - N/A
• Transformation preserves original framework node names - N/A

Validation:

• Unit tests - N/A
• Framework operation tests - N/A
• Transformation tests - N/A
• Model Optimizer IR Reader check - N/A

Documentation:

• Supported frameworks operations list - N/A
• Guide on how to convert the public model - N/A
• User guide update - N/A