Regenerate parser for the observe mini-language (#1743)

This PR regenerates the parser for the `observe` mini-language using the
newest version of Lark. This should fix some deprecation warnings coming
from imports used by the old parser.

The parser was regenerated using Lark 1.1.5, via

python -m lark.tools.standalone traits/observation/_dsl_grammar.lark
--out traits/observation/_generated_parser.py

Closes #1739