Literal `%` in queries should always be escaped.

[dvarrazzo]

Currently in psycopg a lone % is accepted when the query has no parameters, but must be doubled into %% if the query takes parameters. This is confusing because changing a query from non-parametrised to parametrised gives weird errors. Plus, there is issues like #825, which we don't know how to solve.

Requiring to always escape the % is not backward compatible so it won't happen in psycopg2. However it can if we decide to break eggs, so remembering this issue for the mythical psycopg3.

[dvarrazzo]

No, that's not going to happen.