Reduced reduce() usage

django.core.exceptions.ValidationError.messages() and
django.db.backends.schema.BaseDatabaseSchemaEditor._alter_field():
Replaced reduce(operator.add, ...) w/more terse, more explicit sum()

django.core.management.commands.makemigrations.Command.handle_merge():
This reduce() did nothing and would break with larger sequences.
A reduce(operator.eq, ...) accumulates bools and can't generically check
equality of all items in a sequence.