Fixes for Django. migrate and python -> python3.

Applicable spec:

Overview

This PR includes bug fixes for paas-app-charmer. In particular:

Replace erroneous python with python3
If manage.py file exists, use it to run migrations like python3 manage.py migrate in the Django way.

Other necessary fixes (the pipeline was not passing):

Fix Django rockcraft.yaml example thas was missing the base and platforms.
Gunicorn on timeout returns 500 instead of timing out (tests fixed).
Add CVE to trivyignore, as it is included in the current rock.

The version of the library is increased to 1.0.1.

Rationale

This fixes were necessary for a cleaner version of Django applications and also to fix the pipeline.

Juju Events Changes

Module Changes

Library Changes

Checklist

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

Test coverage for 07fd61b6e01cecb6f1c14650540ce08ec8f053ff

Name                                           Stmts   Miss Branch BrPart  Cover   Missing
------------------------------------------------------------------------------------------
paas_app_charmer/__init__.py                       1      0      0      0   100%
paas_app_charmer/_gunicorn/__init__.py             0      0      0      0   100%
paas_app_charmer/_gunicorn/charm.py              117     30     18      6    72%   60-62, 147-148, 150-151, 171->exit, 181-185, 190, 196-204, 208-209, 213, 217, 221, 225, 229, 233, 237, 241, 245, 249
paas_app_charmer/_gunicorn/charm_state.py         63      1      6      1    97%   194
paas_app_charmer/_gunicorn/observability.py       20      0      0      0   100%
paas_app_charmer/_gunicorn/secret_storage.py      14      1      0      0    93%   39
paas_app_charmer/_gunicorn/webserver.py           76      5     16      2    92%   19, 155, 167-173
paas_app_charmer/_gunicorn/wsgi_app.py            62      1     24      1    98%   80
paas_app_charmer/database_migration.py            34      0      2      0   100%
paas_app_charmer/databases.py                     38      3     13      2    90%   82, 97-98
paas_app_charmer/django/__init__.py                2      0      0      0   100%
paas_app_charmer/django/charm.py                  45     13     10      0    65%   72-77, 89, 97-114
paas_app_charmer/exceptions.py                     5      0      0      0   100%
paas_app_charmer/flask/__init__.py                 2      0      0      0   100%
paas_app_charmer/flask/charm.py                   37      0      6      0   100%
paas_app_charmer/helpers.py                        9      0      2      1    91%   18->20
paas_app_charmer/secret_storage.py                40      6     16      4    79%   50, 54->53, 55->57, 85, 103-108
------------------------------------------------------------------------------------------
TOTAL                                            565     60    113     17    87%

Static code analysis report

Run started:2024-05-21 11:25:35.282604

Test results:
    No issues identified.

Code scanned:
    Total lines of code: 1245
    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):

canonical / paas-app-charmer