Assert statements in Enum subclass definitions case errors when assertion rewriting is enabled

While running pytest with the following code:

from enum import Enum

STEP = 100

class SomeEnum(Enum):

    FIRST = 100
    SECOND = FIRST + STEP
    THIRD = SECOND + STEP

    assert THIRD == 300

...assertion rewriting causes an error because of an interaction with Enum:

================================ test session starts ================================
platform darwin -- Python 3.8.12, pytest-6.2.5, py-1.9.0, pluggy-0.13.1
rootdir: /private/tmp/pytest-enum-assert-bug
plugins: profiling-1.7.0, order-1.0.1, cov-3.0.0
collected 0 items / 1 error

====================================== ERRORS =======================================
______________________________ ERROR collecting bug.py ______________________________
bug.py:5: in <module>
    class SomeEnum(Enum):
bug.py:11: in SomeEnum
    assert THIRD == 300
/Users/dsuffling/.pyenv/versions/3.8.12/lib/python3.8/enum.py:112: in __setitem__
    raise TypeError('Attempted to reuse key: %r' % key)
E   TypeError: Attempted to reuse key: '@py_assert1'
============================== short test summary info ==============================
ERROR bug.py - TypeError: Attempted to reuse key: '@py_assert1'
!!!!!!!!!!!!!!!!!!!!!! Interrupted: 1 error during collection !!!!!!!!!!!!!!!!!!!!!!!
================================= 1 error in 0.28s ==================================

This error does not occur with --assert=plain:

================================ test session starts ================================
platform darwin -- Python 3.8.12, pytest-6.2.5, py-1.9.0, pluggy-0.13.1
rootdir: /private/tmp/pytest-enum-assert-bug
plugins: profiling-1.7.0, order-1.0.1, cov-3.0.0
collected 0 items

=============================== no tests ran in 0.02s ===============================

In fact, the issue can be reproduced by:

from enum import Enum

class SomeEnum(Enum):
    assert True

Environment:

pytest version: 6.2.5
environment: macOS 10.14, homebrew 3.3.12, pyenv 2.2.3, CPython 3.8.12
pip list (new virtualenv; nothing but pytest installed):

Package    Version
---------- -------
attrs      21.4.0
iniconfig  1.1.1
packaging  21.3
pip        21.1.1
pluggy     1.0.0
py         1.11.0
pyparsing  3.0.7
pytest     6.2.5
setuptools 56.0.0
toml       0.10.2

Start by reproducing the Enum examples under pytest's assertion-rewriting path, comparing them with --assert=plain. Trace the assertion-rewriting entry point involved during test collection; done means Enum class definitions containing assertions collect without the duplicate @py_assert1 error while ordinary assertion rewriting remains intact.