sagemathgh-39583: Handle various extension degrees in pAdicGenericElement initialisation
    
I have looked into bug sagemath#28555. I have been able to reproduce it with
version 10.6.beta7.

It turns out the infinite loop can be reproduced with:
```
sage: A.<a> = Qq(5^2)
sage: A.base_ring()(a)
```
But the following also triggers the bug: `A.base_ring()(A(1))`. This is
bug sagemath#33527.

The culprit is in the file
`src/sage/rings/padics/padic_template_element.pxi`. Calling
`A.base_ring()(a)` will call `pAdicTemplateElement.__init__` which will
call `A.base_ring()(a)` again on line 140.

To fix it, I added a check for when the degree of the polynomial (over
the base ring $\mathbb Z_p$ or $\mathbb Q_p$) defining the element is
greater than one. If we were trying to convert to the base ring, I raise
a `TypeError` exception. Else, I raise a `NotImplementedError` exception
(this is the case when one tries to convert between two extensions, it
should be implemented later, but here I only fix the bug).

Finally, I change line 140 from `x = self.base_ring()(x)` to `x =
self.base_ring()(x.polynomial().constant_coefficient())` to avoid the
infinite loop.

NOTE: this is my "hello world" PR, I may have broken something with
this, please be kind. Please do not hesitate to tell me how to do
better.

Fixes sagemath#28555.
Fixes sagemath#33527.

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - sagemath#12345: short description why this is a dependency -->
<!-- - sagemath#34567: ... -->

None
    
URL: sagemath#39583
Reported by: Rubén Muñoz--Bertrand
Reviewer(s): Julian Rüth, roed314, Rubén Muñoz--Bertrand

sagemathgh-39583: Handle various extension degrees in pAdicGenericElement initialisation
    
I have looked into bug sagemath#28555. I have been able to reproduce it with
version 10.6.beta7.

It turns out the infinite loop can be reproduced with:
```
sage: A.<a> = Qq(5^2)
sage: A.base_ring()(a)
```
But the following also triggers the bug: `A.base_ring()(A(1))`. This is
bug sagemath#33527.

The culprit is in the file
`src/sage/rings/padics/padic_template_element.pxi`. Calling
`A.base_ring()(a)` will call `pAdicTemplateElement.__init__` which will
call `A.base_ring()(a)` again on line 140.

To fix it, I added a check for when the degree of the polynomial (over
the base ring $\mathbb Z_p$ or $\mathbb Q_p$) defining the element is
greater than one. If we were trying to convert to the base ring, I raise
a `TypeError` exception. Else, I raise a `NotImplementedError` exception
(this is the case when one tries to convert between two extensions, it
should be implemented later, but here I only fix the bug).

Finally, I change line 140 from `x = self.base_ring()(x)` to `x =
self.base_ring()(x.polynomial().constant_coefficient())` to avoid the
infinite loop.

NOTE: this is my "hello world" PR, I may have broken something with
this, please be kind. Please do not hesitate to tell me how to do
better.

Fixes sagemath#28555.
Fixes sagemath#33527.

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - sagemath#12345: short description why this is a dependency -->
<!-- - sagemath#34567: ... -->

None
    
URL: sagemath#39583
Reported by: Rubén Muñoz--Bertrand
Reviewer(s): Julian Rüth, roed314, Rubén Muñoz--Bertrand