pg8000.dbapi is now just pg8000
pg8000 no longer truncates decimals

We can now update the requirements to remove the special case of pg8000
truncating decimals.
Two tests now work for pg800 in EnumTest

The two tests, test_create_table() and test_unicode_labels() previously
failed under pg8000, but now they pass, so this commit opens them up.
pg8000 now passes test_extract() test
Add postgresql+pg8000 to floats_to_four_decimals

pg8000 uses binary transfer of floats and so accuracy is lost for floats
that can't be represented exactly in binary.
Updated doc string for postgresql+pg8000 dialect
Autocommit isolation level for postgresql+pg8000

As with postgresql+psycopg2,
execution_options(isolation_level='AUTOCOMMIT') now works for the
postgresql+pg8000 dialect.

Also enabled the autocommit test in test_dialect.py for pg8000.
PEP 8 tidy of pg8000 dialect and postgresql/test_dialect.py
- changelog + docs for pg8000 transaction isolation level