[MRG] Fix issues in utils.py, code optimisations, readability improvements, improvement in unit test coverage.

[python3-dev]

CHANGES

utils.py

• Fix import-error from pdfminer
• Add type-annotations
• Code optimisations random_string(), remove_extra(), text_strip(), compute_whitespace()
• Improve code-readability.
• Add more unit-test cases.

@vinayak-mehta @bosd: All tests are passing.

[bosd]

@python3-dev Thanks for you contribution. Tests are failling sadly. Can you look into this :pray:

[DoomedJupiter]

@python3-dev the failed test for python 3.8 is due to syntax that was first supported in 3.9 (see here). Possible solutions that involve cutting back on the syntax here