stubgen: fix handling of Protocol and add testcase (#12129)

### Description

This PR fixes #12072 by correctly handling `Protocol` definitions. Previously, the `Protocol` base class was removed when generating type stubs which causes problems with other packages that want to use type definition (because they see it as a regular class, not as a `Protocol`).

## Test Plan

Added a testcase to the stubgen testset.

Co-authored-by: 97littleleaf11 <[email protected]>

mypy/stubgen.py

@@ -851,6 +851,12 @@ def visit_class_def(self, o: ClassDef) -> None:
             base_types.append('metaclass=abc.ABCMeta')
             self.import_tracker.add_import('abc')
             self.import_tracker.require_name('abc')
+        elif self.analyzed and o.info.is_protocol:
+            type_str = 'Protocol'
+            if o.info.type_vars:
+                type_str += f'[{", ".join(o.info.type_vars)}]'
+            base_types.append(type_str)
+            self.add_typing_import('Protocol')
         if base_types:
             self.add('(%s)' % ', '.join(base_types))
         self.add(':\n')

test-data/unit/stubgen.test

@@ -2581,6 +2581,30 @@ def f(x: int, y: int) -> int: ...
 @t.overload
 def f(x: t.Tuple[int, int]) -> int: ...
 
+[case testProtocol_semanal]
+from typing import Protocol, TypeVar
+
+class P(Protocol):
+    def f(self, x: int, y: int) -> str:
+        ...
+
+T = TypeVar('T')
+T2 = TypeVar('T2')
+class PT(Protocol[T, T2]):
+    def f(self, x: T) -> T2:
+        ...
+
+[out]
+from typing import Protocol, TypeVar
+
+class P(Protocol):
+    def f(self, x: int, y: int) -> str: ...
+T = TypeVar('T')
+T2 = TypeVar('T2')
+
+class PT(Protocol[T, T2]):
+    def f(self, x: T) -> T2: ...
+
 [case testNonDefaultKeywordOnlyArgAfterAsterisk]
 def func(*, non_default_kwarg: bool, default_kwarg: bool = True): ...
 [out]