tests: Add integration tests

[mmarseu]

This PR primarily adds integration testing to test the tool's functionality from a "outside" perspective.
These tests invoke the main function directly so they are about as abstract as can be.

The goal is to ensure that the kind of internal redesign as is planned for #7 will not create any regressions. Existing unit tests are not fit for this because they are so tightly coupled to the design that they, too, will have to be reworked, creating the risk of regressions inside the tests.

Tests for the following commands are implemented:

• amend
• build-public
• merge
• set
• validate

Not to be implemented:

• merge-vex, because it is deprecated (see What is the point of merge-vex? #156)

A few minor additional changes

• Disable E704 for flake because it conflicts with black
• Disable bandit on tests because it makes more trouble than it is worth
• Fix an issue in test_customreports.py because I couldn't be bothered to create a separate PR for this. See commit message on 05906d1 for details. If you insist, we can still separate this out.
• Fix a bug that made the --version-range CLI option to the set command not work.

[github-actions]

Coverage Report •

File	Stmts	Miss	Cover	Missing
__main__.py	323	16	95%	217–218, 235, 245, 670, 713, 758, 763, 767, 789, 803, 807, 811, 813, 822, 832
error.py	23	0	100%
log.py	55	1	98%	36
set.py	183	20	89%	56–59, 78–86, 88–89, 91–94, 105
validator
customreports.py	85	9	89%	46–47, 54–57, 97, 149–150
validate.py	96	3	96%	118, 134, 169
TOTAL	1688	71	95%

Tests	Skipped	Failures	Errors	Time
349	2 💤	0 ❌	0 🔥	10.586s ⏱️

[italvi]

@mmarseu some small things, the most "critical" one is the missing license in the header of the new *.py-files 😁