Warn about dict.fromkeys(keys, mutable)

[janosh]

Suggest the following fix when passing a mutable 2nd arg to dict.fromkeys() since modifications will affect all keys which is a common pitfall.

# bad
dict.fromkeys((1, 2), {})
dict.fromkeys((1, 2), [])
dict.fromkeys((1, 2), set())
# ...

# good
{key: {} for key in (1, 2)}
{key: [] for key in (1, 2)}
{key: set() for key in (1, 2)}

Examples:

• StackoverFlow
• SO 2
• commit

Related issue: astral-sh/ruff#4613