sagemathgh-36906: various fixes in quadratic_forms (ruff, pep8, error…

… links, etc)

    
this is fixing various minor details in `quadratic forms`

### 📝 Checklist

- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
    
URL: sagemath#36906
Reported by: Frédéric Chapoton
Reviewer(s): David Coudert

src/sage/quadratic_forms/binary_qf.py

@@ -1928,7 +1928,7 @@ def BinaryQF_reduced_representatives(D, primitive_only=False, proper=True):
     if D > 0:           # Indefinite
         if D.is_square():
             b = D.sqrt()
-            c = ZZ(0)
+            c = ZZ.zero()
             # -b/2 < a <= b/2
             for a in xsrange((-b/2).floor() + 1, (b/2).floor() + 1):
                 if not primitive_only or (gcd([a, b, c]) == 1):

src/sage/quadratic_forms/genera/genus.py

@@ -1202,7 +1202,7 @@ def two_adic_symbol(A, val):
     return [[s[0]+m0] + s[1:] for s in sym + two_adic_symbol(A, val)]
 
 
-class Genus_Symbol_p_adic_ring():
+class Genus_Symbol_p_adic_ring:
     r"""
     Local genus symbol over a `p`-adic ring.
 
@@ -2132,7 +2132,7 @@ def direct_sum(self, other):
 
     def excess(self):
         r"""
-        Returns the `p`-excess of the quadratic form whose Hessian
+        Return the `p`-excess of the quadratic form whose Hessian
         matrix is the symmetric matrix `A`.  When `p = 2`, the `p`-excess is
         called the oddity.
 
@@ -2242,7 +2242,7 @@ def norm(self):
             sage: G = Genus(matrix(ZZ,2,[0, 1, 1, 0]))
             sage: G.local_symbol(2).norm()
             2
-            """
+        """
         if self.rank() == 0:
             return ZZ(0)
         p = self.prime()
@@ -2321,7 +2321,7 @@ def compartments(self):
         return canonical_2_adic_compartments(symbol)
 
 
-class GenusSymbol_global_ring():
+class GenusSymbol_global_ring:
     r"""
     This represents a collection of local genus symbols (at primes)
     and signature information which represent the genus of a

src/sage/quadratic_forms/qfsolve.py

@@ -213,7 +213,7 @@ def solve(self, c=0):
     if not c:
         x = qfsolve(M)
         if isinstance(x, Integer):
-            raise ArithmeticError("no solution found (local obstruction at {})".format(x))
+            raise ArithmeticError(f"no solution found (local obstruction at {x})")
         return x
 
     # If c != 0, define a new quadratic form Q = self - c*z^2
@@ -228,7 +228,7 @@ def solve(self, c=0):
     x = qfsolve(N)
     # Raise an error if qfsolve() doesn't find a solution
     if isinstance(x, Integer):
-        raise ArithmeticError("no solution found (local obstruction at {})".format(x))
+        raise ArithmeticError(f"no solution found (local obstruction at {x})")
 
     # Let z be the last term of x, and remove z from x
     z = x[-1]