remove vendored ibisclient

Make use of upstream PyPI ibisclient rather than vendoring our own. Some things naughtily reach into us to import ucamlookup.ibisclient directly so add a stub module to continue to support that use case.

Freshen up the testing matrix a bit:

• Python 2 has been EoL-ed and we no longer support it.
• Add the supported LTS Django versions. As of writing Django 3.1 is EoL-ed in a week so it doesn't seem worth keeping it in the matrix.
• Remove Python 3.5 as it is EoL-ed.
• Add in Python 3.8 and 3.9.

Closes #16 (closed)