Close files opened with openpyxl

[enekomartinmartinez]

FIles opened with pandas.read_excel are automatically closed, independently of the used engine xlrd for .xls and openpyxl for .xlsx.
However, files opened directly with openpyxl.load_workbook(.., read_only=True) are not. Therefore, a test was added and these files are now closed when Excels.clean() is called.

It was necessary to add psutil dependency for running the tests, as it is used to check which files are opened.

[coveralls]

Coverage increased (+0.003%) to 98.548% when pulling d437b5b on enekomartinmartinez:close_excels into b8a6585 on JamesPHoughton:master.

[enekomartinmartinez]

@JamesPHoughton
However, I think that pandas doesn't automatically close files opened with openpyxl on Windows, see the last comment by a Windows user.
Maybe will be interesting to configure travis to run on Linux, Mac and Windows.

[pep8speaks]

Hello @enekomartinmartinez! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file tests/integration_test_vensim_pathway.py:

Line 76:80: E501 line too long (98 > 79 characters)

Comment last updated at 2021-07-13 12:21:32 UTC

[enekomartinmartinez]

• Correct bug when subseting ranges (added new lines in a tests)
• Correct bug of dynamic final time Dynamic stopping by setting FINAL_TIME #278