Support builtin str removeprefix/removesuffix

These are new in Python 3.9: https://docs.python.org/3/library/stdtypes.html#str.removeprefix

pytype/stubs/builtins/builtins.pytd

@@ -304,6 +304,8 @@ class str(Sequence[str], Hashable):
     def lower(self) -> str: ...
     def lstrip(self, chars: Optional[str] = ...) -> str: ...
     def partition(self, sep: str) -> Tuple[str, str, str]: ...
+    def removeprefix(self, prefix: str) -> str: ...
+    def removesuffix(self, suffix: str) -> str: ...
     def replace(self, old: str, new: str, *args, **kwargs) -> str: ...
     def rfind(self, sub: str, *args, **kwargs) -> int: ...
     def rindex(self, sub: str, *args, **kwargs) -> int: ...

pytype/tests/test_builtins4.py

@@ -286,6 +286,18 @@ def test_str_join(self):
       h = ...  # type: str
     """)
 
+  def test_str_remove_prefix_suffix(self):
+    ty = self.Infer("""
+      a = "prefix_suffix"
+      b = a.removeprefix("prefix_")
+      c = a.removesuffix("_suffix")
+    """, deep=False)
+    self.assertTypesMatchPytd(ty, """
+      a = ...  # type: str
+      b = ...  # type: str
+      c = ...  # type: str
+    """)
+
   def test_str_is_hashable(self):
     self.Check("""
       from typing import Any, Dict, Hashable