ipympl changes MPL backend on import

• xref: https://github.com/mne-tools/mne-python/pull/13963#pullrequestreview-4499976618

In mne-python (see above), we have a functionality called sys_info, which gives us (among other things) the versions of all sorts of installed packages.

For this reason, we also import ipympl.

At import time, ipympl actually changes the MPL backend.

Is this something that could be avoided? This change in backend was very surprising to us and it took an in-depth profiling to identify it as the cause of a massive memory leak in a pipeline that produced several plots on each iteration, never closing them due to the backend change.

检查 issue 和链接的 mne-python pull request review 中描述的导入时行为，重点关注导入 ipympl 会在何处改变 Matplotlib backend。使用 sys_info 风格的导入序列重现这一情况，并验证导入 ipympl 不再改变 backend，也不会导致报告中的 plot 生命周期问题。