Fix #106 -- discard alpha channel on PNGs

[amureki]

@codingjoe this is a very simple draft made to start a discussion and see your opinion on how things should be handled here. Questions to consider:

• do we want to discard alpha channel or allow users to select a handling strategy?
• should we log this situation as a warning?
• shall we consider other modes, like "P" or "LA"?

[codecov[bot]]

Codecov Report

Patch coverage: 80.00% and project coverage change: -0.23 :warning:

Comparison is base (c295bcb) 100.00% compared to head (3b503e9) 99.77%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #107 +/- ## =========================================== - Coverage 100.00% 99.77% -0.23% =========================================== Files 13 13 Lines 443 448 +5 =========================================== + Hits 443 447 +4 - Misses 0 1 +1 ``` | Flag | Coverage Δ | | |---|---|---| | celery | `85.49% <80.00%> (-0.07%)` | :arrow_down: | | cleanup | `86.16% <80.00%> (-0.07%)` | :arrow_down: | | dj4.0 | `86.16% <80.00%> (-0.07%)` | :arrow_down: | | dj4.1 | `86.16% <80.00%> (-0.07%)` | :arrow_down: | | dramatiq | `85.49% <80.00%> (-0.07%)` | :arrow_down: | | drf | `93.75% <80.00%> (-0.16%)` | :arrow_down: | | macos-latest | `86.38% <80.00%> (-0.08%)` | :arrow_down: | | py3.10 | `86.16% <80.00%> (-0.07%)` | :arrow_down: | | py3.11 | `86.16% <80.00%> (-0.07%)` | :arrow_down: | | py3.8 | `86.16% <80.00%> (-0.07%)` | :arrow_down: | | py3.9 | `86.16% <80.00%> (-0.07%)` | :arrow_down: | | ubuntu-latest | `86.16% <80.00%> (-0.07%)` | :arrow_down: | | windows-latest | `78.76% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Johannes+Maron#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://codecov.io/gh/codingjoe/django-pictures/pull/107?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Johannes+Maron) | Coverage Δ | | |---|---|---| | [pictures/models.py](https://codecov.io/gh/codingjoe/django-pictures/pull/107?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Johannes+Maron#diff-cGljdHVyZXMvbW9kZWxzLnB5) | `99.24% <80.00%> (-0.76%)` | :arrow_down: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Johannes+Maron). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Johannes+Maron)

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[codingjoe]

As we discussed, I took the time to address the issue. Thanks for your patch anyways :)