TextBox does not accept dead keys

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### Bug report

**Bug summary**

Typing a dead key (a key which is not written directly after a keypress, e.g. ^, ´, `) into a TextBox widget raises a TypeError. This also occurs when you press CapsLock. This prevents the user from entering some characters like "^" or using CapsLock.

Edit: After about version `3.4` no error is being raised. Now CapsLock works but the TextBox just ignores dead keys.

**Code for reproduction**

```python
from matplotlib import pyplot as plt
from matplotlib.widgets import TextBox

fig, ax = plt.subplots(figsize=(6, 3))
fig.subplots_adjust(bottom=0.2)
text_box_ax = fig.add_axes([0.4, 0.02, 0.5, 0.06])
text_box = TextBox(text_box_ax, "Type diacritic character (e.g. ^): ")
plt.show()
```

**Actual outcome**

```
Traceback (most recent call last):
File "\matplotlib\cbook\__init__.py", line 196, in process
func(*args, **kwargs)
File "\matplotlib\widgets.py", line 790, in _keypress
if len(key) == 1:
TypeError: object of type 'NoneType' has no len()
```

**Expected outcome**
Normal behaviour of those characters when typing them in editors or searchbars etc.

**Matplotlib version**

* Operating system: windows 10, build 19041.867
* Matplotlib version: `3.2.2`
* Matplotlib backend: `TkAgg`
* Python version: 3.8
* Jupyter version (if applicable):
* Other libraries:

Commencez par le gestionnaire TextBox._keypress dans matplotlib/widgets.py et exécutez la reproduction fournie de TextBox avec le backend TkAgg. Suivez la manière dont les touches mortes et CapsLock sont représentées, puis vérifiez que la saisie de caractères diacritiques ne provoque plus d’erreur et n’ignore plus silencieusement la touche, et que l’édition ordinaire de TextBox fonctionne toujours.

Rédigé par le modèle d'indexation à partir du texte de l'issue.