Merge "Add collation_schema parameter for schema-qualified PostgreSQL collations...

diff --cc doc/build/changelog/migration_21.rst
Simple merge

diff --cc lib/sqlalchemy/dialects/postgresql/base.py
Simple merge

diff --cc lib/sqlalchemy/sql/_elements_constructors.py
Simple merge

diff --cc lib/sqlalchemy/sql/compiler.py
Simple merge

diff --cc lib/sqlalchemy/sql/sqltypes.py
Simple merge

diff --cc test/dialect/postgresql/test_compiler.py
Simple merge

diff --cc test/sql/test_types.py
Simple merge