Delete unused files replaced by pyproject.toml and update README

[jjmerchante]

This PR:

• Removes unused files like requirements.txt, setup.py, and setup.cfg. Now we only use pyproject.toml. Includes .flake8 as a replacement for setup.cfg.
• Updates the documentation to install the package.

Related to https://github.com/chaoss/grimoirelab/issues/532

[coveralls]

Pull Request Test Coverage Report for Build 3280386808

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.3%) to 82.366%

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Totals
Change from base Build 3272832390:	-0.3%
Covered Lines:	8842
Relevant Lines:	10735

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💛 - Coveralls