Lazy postgresql.exceptions

[jwp]

The number of classes in postgresql.exceptions adds measurable overhead to the startup time of processes importing postgresql. Constructing the exception classes upon use would eliminate this initial overhead and memory footprint (CPython types being relatively large).

[jwp]

Python 3.7 adds getattr/dir support to modules. This should allow a portable means for managing lazy exception creation. This should probably be a 2.0 feature that adds a 3.7 requirement.

[jwp]

Might be wise to wait until https://bugs.python.org/issue12029

Exceptions could have an SQLSTATE code physical hierarchy and ABC based groups.