Remove _ prefix from TypeVars which appear in public API

The prefixes make the API Reference docs (for e.g. `pytest.raises`,
`pytest.fixture`) uglier.

Being under `_pytest` is sufficient from a privacy perspective, so let's
drop them.

src/_pytest/_code/code.py

@@ -436,26 +436,26 @@ co_equal = compile(
 )
 
 
-_E = TypeVar("_E", bound=BaseException, covariant=True)
+E = TypeVar("E", bound=BaseException, covariant=True)
 
 
 @final
 @attr.s(repr=False)
-class ExceptionInfo(Generic[_E]):
+class ExceptionInfo(Generic[E]):
     """Wraps sys.exc_info() objects and offers help for navigating the traceback."""
 
     _assert_start_repr = "AssertionError('assert "
 
-    _excinfo = attr.ib(type=Optional[Tuple[Type["_E"], "_E", TracebackType]])
+    _excinfo = attr.ib(type=Optional[Tuple[Type["E"], "E", TracebackType]])
     _striptext = attr.ib(type=str, default="")
     _traceback = attr.ib(type=Optional[Traceback], default=None)
 
     @classmethod
     def from_exc_info(
         cls,
-        exc_info: Tuple[Type[_E], _E, TracebackType],
+        exc_info: Tuple[Type[E], E, TracebackType],
         exprinfo: Optional[str] = None,
-    ) -> "ExceptionInfo[_E]":
+    ) -> "ExceptionInfo[E]":
         """Return an ExceptionInfo for an existing exc_info tuple.
 
         .. warning::
@@ -500,17 +500,17 @@ class ExceptionInfo(Generic[_E]):
         return ExceptionInfo.from_exc_info(exc_info, exprinfo)
 
     @classmethod
-    def for_later(cls) -> "ExceptionInfo[_E]":
+    def for_later(cls) -> "ExceptionInfo[E]":
         """Return an unfilled ExceptionInfo."""
         return cls(None)
 
-    def fill_unfilled(self, exc_info: Tuple[Type[_E], _E, TracebackType]) -> None:
+    def fill_unfilled(self, exc_info: Tuple[Type[E], E, TracebackType]) -> None:
         """Fill an unfilled ExceptionInfo created with ``for_later()``."""
         assert self._excinfo is None, "ExceptionInfo was already filled"
         self._excinfo = exc_info
 
     @property
-    def type(self) -> Type[_E]:
+    def type(self) -> Type[E]:
         """The exception class."""
         assert (
             self._excinfo is not None
@@ -518,7 +518,7 @@ class ExceptionInfo(Generic[_E]):
         return self._excinfo[0]
 
     @property
-    def value(self) -> _E:
+    def value(self) -> E:
         """The exception value."""
         assert (
             self._excinfo is not None