Reverse loosening of `priority` constraints in regex & tests

[bskinn]

Per discussion at #181, Sphinx will silently ignore object entries with non-integer priority values

• [x] Code
• [x] Tests
  • [x] Remove objects.inv files with non-integer priority from main tests
  • ~Establish consistent nomenclature to identify objects.inv with non-integer priority and add fails-to-load tests for them~
  • [x] More complicated than necessary. With the new custom-built data line testing, it was easy to put invalid priority cases in.
• [x] Docs
  • Handled a/o 6632dd4
• [x] CHANGELOG