Enable collection of branch coverage on test runs

[devdanzin]

Having branch coverage information helps when trying to address issues like #698 and #629 (where we used it to find places to test).

This PR adds the --branch option to coverage run in tox.ini commands.

Possible downsides: test run time will likely increase, coverage numbers will probably go down. Still, the difference should not be huge and it will help improving coverage in the long term.

[coveralls]

Pull Request Test Coverage Report for Build 10102688154

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 97.492%

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Totals
Change from base Build 10100578761:	-0.1%
Covered Lines:	3176
Relevant Lines:	3254

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💛 - Coveralls

[niccokunzmann]

Cool, thanks!