gh-36071: Correct typo in class name of `Polynomial_ZZ_pEX` in `.pxd` file
    
<!-- ^^^^^
Corrects a typo in the cython definition of Polynomial_ZZ_pEX
-->
Currently the cpython definition of within
`/sage/rings/polynomial/polynomial_zz_pex.pxd` exports the class name
`Polynomial_ZZ_pX` instead of `Polynomial_ZZ_pEX`.

At the moment, if a user requires the `Polynomial_ZZ_pEX` within a
`.pyx` file, they must perform the following import:

```
from sage.rings.polynomial.polynomial_zz_pex cimport Polynomial_ZZ_pX as
Polynomial_ZZ_pEX
```

Correcting the typo in the changed file allows the import to be simply

```
from sage.rings.polynomial.polynomial_zz_pex cimport Polynomial_ZZ_pEX
```

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URL: #36071
Reported by: Giacomo Pope
Reviewer(s): Frédéric Chapoton

Author: Release Manager

src/sage/rings/polynomial/polynomial_zz_pex.pxd

@@ -6,6 +6,6 @@ ctypedef ZZ_pEContext_ptrs *cparent
 
 include "polynomial_template_header.pxi"
 
-cdef class Polynomial_ZZ_pX(Polynomial_template):
+cdef class Polynomial_ZZ_pEX(Polynomial_template):
     pass