implement per-dialect datetime representations, not just strsqlcompiler

When I try to compile a query with datetime parameters using literal_binds, e.g.:

from sqlalchemy import create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, DateTime
from sqlalchemy.orm import sessionmaker
from datetime import datetime

engine = create_engine('sqlite:///:memory:', echo=True)
Base = declarative_base()
class M(Base):
    __tablename__ = 'M'
    d = Column(DateTime, primary_key=True)

Session = sessionmaker()

print(Session().query(M).filter(
    M.d < datetime.now()
).statement.compile(
    compile_kwargs={'literal_binds': True}
))

the engine gives me the following error

NotImplementedError: Don't know how to literal-quote value datetime.datetime(2019, 12, 25, 14, 26, 42, 756674)

There probably should be a default representation for datetimes as an iso-formatted string in single quotes.

[CaselIT]

I think that we can reopen this, since at the moment we just implemented a default representation when literally compile without a dialect. These is still no implementation for specific dialects, other that sqlite

[sqla-tester]

Mike Bayer has proposed a fix for this issue in the main branch:

implement iso date literals for all backends https://gerrit.sqlalchemy.org/c/sqlalchemy/sqlalchemy/+/3744