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:

Start with the TextBox._keypress handler in matplotlib/widgets.py and run the provided TextBox reproduction using the TkAgg backend. Trace how dead keys and CapsLock are represented, then verify that typing diacritics no longer raises an error or silently ignores the key and that ordinary TextBox editing still works.

Written by the indexing model from the issue text.