fix: tell type checkers that the config options are strings (trivial)

[tonyandrewmeyer]

Applicable spec: N/A

Overview

ops<=2.12 wrongly has self.config[x] typed as str, when actually it could be an int, float, bool, or str, depending on the config type. We're fixing this in ops:#1183, but that will break static checking that currently assumes that the config is a str (because ops doesn't validate the schema, so all options will be bool|int|float|str).

This PR adds a typing.cast call where the config values are loaded and used where the type should be str.

Rationale

Covered in the overview.

Juju Events Changes

N/A

Module Changes

N/A

Library Changes

N/A

Checklist

• The charm style guide was applied
• The contributing guide was applied
• The changes are compliant with ISD054 - Managing Charm Complexity
• The documentation is generated using src-docs
• The documentation for charmhub is updated
• The PR is tagged with appropriate label (urgent, trivial, complex)

No doc changes.

[yanksyoon]

/canonical/self-hosted-runners/run-workflows 8554612

[github-actions]

Test coverage for 8554612

Name           Stmts   Miss Branch BrPart  Cover   Missing
----------------------------------------------------------
src/charm.py     188      3     68      5    97%   302->305, 396, 403->391, 409, 417
----------------------------------------------------------
TOTAL            188      3     68      5    97%

Static code analysis report

Run started:2024-04-11 06:26:19.315047

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 1059
  Total lines skipped (#nosec): 0
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 0

Run metrics:
  Total issues (by severity):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
  Total issues (by confidence):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
Files skipped (0):

[arturo-seijas]

Hi! I think I've done and merge the same change in #99. Thanks for the contribution in any case :)