`__init__.py`: fix formatting of constants

[cmoussa1]

Problem

The constants defined in __init__.py for the flux-accounting Python package are formatted incorrectly and are flagged by pylint. They should be in all caps:

__init__.py:1:0: C0103: Constant name "db_dir" doesn't conform to '(([A-Z_][A-Z0-9_]*)|(__.*__))$' pattern (invalid-name)
__init__.py:2:0: C0103: Constant name "db_path" doesn't conform to '(([A-Z_][A-Z0-9_]*)|(__.*__))$' pattern (invalid-name)
__init__.py:3:0: C0103: Constant name "db_schema_version" doesn't conform to '(([A-Z_][A-Z0-9_]*)|(__.*__))$' pattern (invalid-name)

---

This PR fixes the format of the constants defined in __init__.py and changes the references to these constants throughout the Python package to account for the change.

[cmoussa1]

Thanks @jameshcorbett! setting MWP here