Fix E731

[harupy]

E731 should be only emitted when the assignment target is a Name.

a = lambda: None  # bad, should be converted to `def a(): return None`.
a.b = lambda: None  # ok
a["b"] = lambda  # ok

# Flake8

> flake8 --select E731 resources/test/fixtures/E731.py
resources/test/fixtures/E731.py:2:1: E731 do not assign a lambda expression, use a def
resources/test/fixtures/E731.py:4:1: E731 do not assign a lambda expression, use a def
resources/test/fixtures/E731.py:7:5: E731 do not assign a lambda expression, use a def

# Current

> cargo run -- --select E731 resources/test/fixtures/E731.py
resources/test/fixtures/E731.py:2:1: E731 Do not assign a lambda expression, use a def
resources/test/fixtures/E731.py:4:1: E731 Do not assign a lambda expression, use a def
resources/test/fixtures/E731.py:7:5: E731 Do not assign a lambda expression, use a def
resources/test/fixtures/E731.py:12:1: E731 Do not assign a lambda expression, use a def  👈
resources/test/fixtures/E731.py:16:1: E731 Do not assign a lambda expression, use a def  👈

# With fix

> cargo run -- --select E731 resources/test/fixtures/E731.py
resources/test/fixtures/E731.py:2:1: E731 Do not assign a lambda expression, use a def
resources/test/fixtures/E731.py:4:1: E731 Do not assign a lambda expression, use a def
resources/test/fixtures/E731.py:7:5: E731 Do not assign a lambda expression, use a def