v0.20.0

Added

• A lone % in a sql= statement is refused at declaration time. The
  statement is run with its parameters, and an empty parameter sequence is still
  a sequence, so a % that is not a placeholder is read as the start of one:
  the failure came from psycopg/cursor.py at build time, naming the driver and
  nothing about the shape. A caller who passed no parameters at all had no model
  that explained it, and the modulo operator is the reason anyone writes a bare
  % in the first place.

  A valid statement can never contain one, so nothing legal is refused -- which
  is the whole reason this can be a refusal rather than an escape. sql= takes
  params=, so pyformat is its interface and %% stays the spelling for the
  operator.

  SqlValue on the derived path escapes instead, and the two are not
  inconsistent: there the caller supplies no parameters and has no reason to
  know one exists. Both docstrings and the projections guide now say which is
  which, at both call sites, because the asymmetry is exactly what a reader
  finding one of them will next be confused by.