Use "is" to compare __class__

Since the value of __class__ is a type, comparing it to another
type object should use "is" rather than "==".

Some of these, involving type(), were fixed in bf7af69, but flake8
did not catch the .__class__ variation addressed here.

git/refs/symbolic.py

@@ -769,7 +769,7 @@ def iter_items(
 
             List is lexicographically sorted
             The returned objects represent actual subclasses, such as Head or TagReference"""
-        return (r for r in cls._iter_items(repo, common_path) if r.__class__ == SymbolicReference or not r.is_detached)
+        return (r for r in cls._iter_items(repo, common_path) if r.__class__ is SymbolicReference or not r.is_detached)
 
     @classmethod
     def from_path(cls: Type[T_References], repo: "Repo", path: PathLike) -> T_References:
@@ -797,7 +797,7 @@ def from_path(cls: Type[T_References], repo: "Repo", path: PathLike) -> T_Refere
             try:
                 instance: T_References
                 instance = ref_type(repo, path)
-                if instance.__class__ == SymbolicReference and instance.is_detached:
+                if instance.__class__ is SymbolicReference and instance.is_detached:
                     raise ValueError("SymbolicRef was detached, we drop it")
                 else:
                     return instance