Add Photo.getSafetyLevel() (#173)

[beaufour]

Closes #173.

Problem

There is no flickr.photos.getSafetyLevel endpoint, so it wasn't obvious how to read a photo's safety level (e.g. to find photos with a given tag, check the level, and fix it if wrong).

Solution

The safety level is returned by flickr.photos.getInfo (and by search with extras="safety_level") — it just wasn't surfaced conveniently. This PR:

• Adds a Photo.getSafetyLevel() helper that loads the photo if needed and returns the level as an int (0=none, 1=safe, 2=moderate, 3=restricted) — matching the values setSafetyLevel accepts.
• Registers an int converter for safety_level so photo.safety_level is a consistent integer however the photo was fetched.

Usage

# Efficient bulk check via search extras (no per-photo call)
for p in flickr_api.Photo.search(tags="mytag", extras="safety_level"):
    if p.getSafetyLevel() != 3:
        p.setSafetyLevel(safety_level=3)

Tests

Adds tests covering the int conversion via getInfo, getSafetyLevel() triggering a load when the value is unknown, and getSafetyLevel() avoiding a call when the value is already present. Full suite (285 tests), mypy, and flake8 all pass.

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