pretty formatted output and calculation for `lines-valid` and `lines-covered`

[nickv2002]

Add

1. pretty formatted output
2. calculation for lines-valid and lines-covered
3. Black formatting

Note this assumes Python 3 (since Python 2 should not be used anymore) and incorporates changes from https://github.com/rix0rrr/cover2cover/pull/13 to fix the byte output problem also.