gh-40415: Remove precisely the prefix instead of strip
    
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Hello,

I am a first time contributor, still navigating the `sage` huge
codebase. Please let me know if I am not aware of something.

This PR resolves a few occurrences of the error
[B005](https://docs.astral.sh/ruff/rules/strip-with-multi-characters/)
from `ruff`.

It looks pretty clear to me that that intent of the code is to remove
precisely these prefixes rather than, for example, removing all the
leading occurrences of the letters d, e, f, a, u, l, t. I think the
example in the docs of `ruff` illustrates quite well the difference.

Tested with `ruff check src/sage --select B005`

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- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

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URL: #40415
Reported by: Trinh Quoc Anh
Reviewer(s): Frédéric Chapoton

Author: Release Manager

src/sage/interfaces/singular.py

@@ -1654,7 +1654,7 @@ def sage_global_ring(self):
             is_extension = False
         else:
             # it ought to be a finite field
-            q = ZZ(charstr[0].lstrip('ZZ/'))
+            q = ZZ(charstr[0].removeprefix('ZZ/'))
             from sage.rings.finite_rings.finite_field_constructor import GF
             if q.is_prime():
                 br = GF(q)

src/sage/plot/misc.py

@@ -420,20 +420,20 @@ def get_matplotlib_linestyle(linestyle, return_type):
         return None
 
     if linestyle.startswith("default"):
-        return get_matplotlib_linestyle(linestyle.strip("default"), "short")
+        return get_matplotlib_linestyle(linestyle.removeprefix("default"), "short")
     elif linestyle.startswith("steps"):
         if linestyle.startswith("steps-mid"):
             return "steps-mid" + get_matplotlib_linestyle(
-                linestyle.strip("steps-mid"), "short")
+                linestyle.removeprefix("steps-mid"), "short")
         elif linestyle.startswith("steps-post"):
             return "steps-post" + get_matplotlib_linestyle(
-                linestyle.strip("steps-post"), "short")
+                linestyle.removeprefix("steps-post"), "short")
         elif linestyle.startswith("steps-pre"):
             return "steps-pre" + get_matplotlib_linestyle(
-                linestyle.strip("steps-pre"), "short")
+                linestyle.removeprefix("steps-pre"), "short")
         else:
             return "steps" + get_matplotlib_linestyle(
-                linestyle.strip("steps"), "short")
+                linestyle.removeprefix("steps"), "short")
 
     if return_type == 'short':
         if linestyle in short_to_long_dict.keys():