Fix bugs in the code-coverage report

[WenjieDu]

This PR is to fix previous bugs the testing cases in test_cli.py that are

1. deleting .coverage file causes the mis-calculation of the code coverage;
2. linting code with Black reformats the code and results in discrepancy between records in the .coverage file and actual code;

Both fixed in this PR.

[coveralls]

Pull Request Test Coverage Report for Build 4778543567

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 11 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.2%) to 84.641%

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Files with Coverage Reduction	New Missed Lines	%
pypots/cli/doc.py	3	82.05%
pypots/tests/test_cli.py	3	90.91%
pypots/cli/dev.py	5	71.67%

Totals
Change from base Build 4777707592:	-0.2%
Covered Lines:	3053
Relevant Lines:	3607

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💛 - Coveralls