Recognize a class as protocol if at least one of the bases is Protocol (#7455)

Before only the last base might take effect in some situations.

Fixes #7381

Author: thomaslee

mypy/semanal.py

@@ -1227,7 +1227,8 @@ class Foo(Bar, Generic[T]): ...
                 if declared_tvars:
                     self.fail('Only single Generic[...] or Protocol[...] can be in bases', context)
                 removed.append(i)
-                tvars, is_protocol = result
+                tvars = result[0]
+                is_protocol |= result[1]
                 declared_tvars.extend(tvars)
             if isinstance(base, UnboundType):
                 sym = self.lookup_qualified(base.name, base)

test-data/unit/check-semanal-error.test

@@ -97,3 +97,19 @@ def m() -> None: ...  # E: Name 'm' already defined (possibly by an import)
 def f() -> None: ...  # E: Name 'f' already defined (possibly by an import)
 
 [out]
+
+[case testRuntimeProtoTwoBases]
+from typing_extensions import Protocol, runtime_checkable
+from typing import TypeVar, Generic
+
+T = TypeVar('T')
+
+@runtime_checkable
+class P(Protocol, Generic[T]):
+    attr: T
+
+class C:
+    attr: int
+
+x: P[int] = C()
+[out]