Bamnostic

Changed relative path of `bamnostic.AlignmentFile` index file lookup to absolute path.

Thanks to some community contributions, minor quality of life improvements have been added:
* 1-length sequences created as artifacts from some platforms can now be handled correctly (giesselmann)
* Any unnamed sequence is now given a default `.reference_name` attribute of `"*"` (OliverVoogd)

Appveyor does not support Pytest on Python 3.9. Will debug later

Includes versioning for Python 3.7-3.9

There was a typo in v1.1.1- that affected how `bamnostic.bam.Bam` would process BAI files that was missing `n_no_coor` at the end of it. Up till now, `bamnostic` would set these to None. But in places where `bamnostic.bam.Bam` referenced this (`bamnostic.bam.Bam._init_index` and the `bamnostic.bam.Bam.unmapped` property) it couldn't add (`+`) the mapped and unmapped reads to `n_no_coord`. This would result in an error since a mathematical operation between an integer and NoneType doesn't work.