activate_matplotlib() is not called.

## Summary

Unless you intentionally omitted it, It seems to need to call ``IPython.core.pylabtools.activate_matplotlib()`` function in ``_enable_matplotlib_integration()``.

## Details

[Since version 4.4.0](https://github.com/ipython/ipykernel/blob/523cb549add6e2d5984a831582eaabd9c9af6aa3/docs/changelog.rst#440), the default backend has been changed to ``inline`` (``module://ipykernel.pylab.backend_inline``). In this issue, it's called *default inline mode*.

Consequently, we no longer need to specify inline mode using `%matplotlib inline` with Jupyter Notebook. The mode when using the magic command is called *explicit inline mode*.

However, I've realized that the behavior of default inline mode and that of explicit inline mode are different.

1. the behavior of default inline mode

```python
from matplotlib import pyplot as plt

x = [1, 2, 3]
y = [3, 6, 9]

fig = plt.figure()
ax = fig.add_subplot(111)
ax.set_title('Sample Bar Chart')
```


The figure is not rendered when a code-cell is executed. To render a graph, I have to call``plt.show()``.

2. In the case of explicitly inline mode.

```python
from matplotlib import pyplot as plt
%matplotlib inline

x = [1, 2, 3]
y = [3, 6, 9]

fig = plt.figure()
ax = fig.add_subplot(111)
ax.set_title('Sample Bar Chart')
```


The figure is rendered even if ``plot.show()`` is not executed.

I don't know whether these behaviors are correct or not. On the other hand, I think that users are confused by it.

## Proposals

When ``%matplotlib inline`` is called, ``configure_inline_support()`` function and ``activate_matplotlib()`` are executed [^1] . But in `_enable_matplotlib_integration()`, ``activate_matplotlib()`` is not called.

Therefor, maybe simply it should be called in the Initialization process.

Could you tell me your opinion?

[^1]: https://github.com/ipython/ipython/blob/3be8fc96b227125f2dc79fc881f77b5b4e9652fb/IPython/core/interactiveshell.py#L2919