Make smtp charm library compatible with Python 3.8

[weiiwang01]

Applicable spec:

Overview

Make the smtp charm library compatible with Python 3.8 to pass the interface test

Fix: canonical/charm-relation-interfaces#189

Rationale

Juju Events Changes

Module Changes

Library Changes

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)

[github-actions]

Test coverage for 3f8e386

Name                                    Stmts   Miss Branch BrPart  Cover   Missing
-----------------------------------------------------------------------------------
lib/charms/smtp_integrator/v0/smtp.py     130      2     22      2    97%   340, 342
src/charm.py                               82      5     26      1    94%   91-93, 102-103
src/charm_state.py                         46      0      0      0   100%
-----------------------------------------------------------------------------------
TOTAL                                     258      7     48      3    97%

Static code analysis report

Run started:2024-11-12 16:57:55.317496

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 926
  Total lines skipped (#nosec): 3
  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):