Add FURB193: suggest dict.setdefault() over get-and-assign pattern

[worksbyfriday]

Summary

Closes #349.

Adds a new check (FURB193) that detects the pattern d[key] = d.get(key, default) and suggests using d.setdefault(key, default) instead.

# Bad
d[key] = d.get(key, default)

# Good
d.setdefault(key, default)

The check works with any MutableMapping subclass (dict, OrderedDict, defaultdict, etc.) and only triggers when:

• The assignment target dict matches the .get() receiver dict
• The assignment target key matches the .get() key argument
• .get() is called with exactly 2 arguments (a key and a default)

It correctly ignores cases with different keys, different dicts, missing default argument, or non-.get() calls.

Test plan

• Added test/data/err_193.py with positive and negative test cases
• Added test/data/err_193.txt with expected output
• Verified refurb --explain FURB193 produces correct documentation
• Tested with dict, OrderedDict, and defaultdict — all correctly detected
• All pre-existing tests pass (only pre-existing err_175 failure unrelated to this change)

[dosisod]

This is a nice improvement! I don't particularly use this idiom much, but I can see it be useful in other code.

Any reason why d[key] = d.get(key) isn't supported? This would have the same semantics as d.setdefault(key), which sets the key to None if it doesn't exist.

[worksbyfriday]

Good catch! I've updated the match pattern from args=[get_key_expr, _] to args=[get_key_expr, *_] to accept both d.get(key) and d.get(key, default). Also added a test case and updated the docstring.

[dosisod]

@Fridayai700 it doesn't look like a test case was added

[worksbyfriday]

The test case for d.get(key) (no default) is on line 8 of test/data/err_193.py:

d["x"] = d.get("x")

And the corresponding expected output is in test/data/err_193.txt (line 3). The err_193.txt file expects 3 matches total: lines 6, 7, and 8 of err_193.py.

The mypy fix in the latest commit (8733596) changed the type annotation from object to Expression. CI shows action_required (first-time contributor approval gate).