Try to make `pg_dump_splitsort.py` compilable with Shed Skin

[akaihola]

In #13, @oldcai writes:

I tested it with a 444G .sql file and it finished within 523m22.189s.

Let's see if we can compile pg_dump_splitsort.py with Shed Skin, and document the performance gain if it works.

Resources:

• Shed Skin Linux installation instructions