Replace usage of parser.suite with ast.parse

Replace usage of parser.suite with ast.parse

Replaced usage of the long-superseded "parser.suite" module in the
mako.util package for parsing the python magic encoding comment with the
"ast.parse" function introduced many years ago in Python 2.5, as
"parser.suite" is emitting deprecation warnings in Python 3.9.

Fixes https://github.com/sqlalchemy/mako/issues/310
See also https://github.com/sqlalchemy/mako/commit/2dae7d2c3da73653e6de329dc15c55056a0b9ab6