it back to a string (will raise an error if it's actually non-ASCII).

Conflicts:
	pymbolic/interop/sympy.py

Flake8 during ci

See merge request !2

Run Flake8 during CI

See merge request !1

More unifier fixes.

unify().

Fixes to unification records:
  - Properly merge rmap.
  - Don't store redundant equations.

Other fixes:
  - Fix an issue where existing urecs weren't properly being merged
    in map_commut_assoc().

Fix avoidable exponential blowup in unifier.