`unit_size` does not work in Axes

## Description of bug / unexpected behavior

The **Axes** class does not respect the `"unit_size"` setting with the `x_axis_config` or `y_axis_config` parameter.

## Expected behavior

If you change **Axes** to **NumberPlane** then it solves:

```python
class Test1(Scene):
def construct(self):
axes = NumberPlane(
x_range=[-3,3,1],
y_range=[-6,6,2],
x_axis_config={
"unit_size": 1
},
y_axis_config={
"unit_size": 0.5
}
)
self.add(axes)
```


## How to reproduce the issue
```python
class AxesBug(Scene):
def construct(self):
axes = Axes(
x_range=[-3,3,1],
y_range=[-6,6,2],
x_axis_config={
"unit_size": 1
},
y_axis_config={
"unit_size": 0.5
}
)
self.add(axes)
```

## My specs.

Manim 0.12, tested on MacOS M1 Big Sur and Linux Mint 20.1

Start with the Axes and NumberPlane entry points and reproduce the example using x_axis_config and y_axis_config with unit_size. Compare how the two classes handle these settings, then verify that Axes respects the requested x- and y-axis unit sizes in the same example.

Written by the indexing model from the issue text.