Update to new LayerPotentialBase

See merge request inducer/pytential!92

Try hard to find a git revision to use as cache key

See merge request inducer/pytential!93

Merge branch 'use-git-rev-as-cache-key' of ssh://gitlab.tiker.net/inducer/pytential into use-git-rev-as-cache-key

# Conflicts:
#   pytential/version.py

Use sumpy and boxtree with compressed list 3

See merge request inducer/pytential!91

Remove now-extraneous 'nosync' specs now that loopy got smarter

See merge request !90

Do not oversample scalars

See merge request inducer/pytential!89