Fix traceback.FrameSummary's "line" parameter

traceback.FrameSummary's "line" parameter is a string that is the text
of a line of code, not an int that is the line number of a line of
code.

Also relax type of traceback.format_list's "extracted_list" parameter
in 3.5-and-later from List[FrameSummary] to Iterable[FrameSummary].
Never ask for a List when asking for a Sequence will do, and never ask
for a Sequence when asking for a Collection will do, and never ask for
a Collection when asking for an Iterable will do (and so on in other
cases, but this case stops at Iterable).

Also add a note about deliberately not supporting for type-checking
purposes the 3.4-and-earlier List[Tuple[str, int, str, Optional[str]]]
semantics for traceback.format_list's "extracted_list" parameter. The
traceback module currently supports the old type of parameter, but if
authors are typechecking their 3.5-and-later code, it's reasonable to
expect that they'll want to modernize their code to use FrameSummary
objects rather than four-tuples.

Author: nathanielmanistaatgoogle

stdlib/2and3/traceback.pyi

@@ -1,6 +1,6 @@
 # Stubs for traceback
 
-from typing import Any, Dict, Generator, IO, Iterator, List, Mapping, Optional, Tuple, Type, Iterable
+from typing import Any, Dict, Generator, IO, Iterable, Iterator, List, Mapping, Optional, Tuple, Type
 from types import FrameType, TracebackType
 import sys
 
@@ -35,7 +35,12 @@ if sys.version_info >= (3, 5):
     def extract_tb(tb: Optional[TracebackType], limit: Optional[int] = ...) -> StackSummary: ...
     def extract_stack(f: Optional[FrameType] = ...,
                       limit: Optional[int] = ...) -> StackSummary: ...
-    def format_list(extracted_list: List[FrameSummary]) -> List[str]: ...
+    # NOTE(nathaniel): extracted_list *could* be of type
+    # "Iterable[Union[_PT, FrameSummary]]", but if authors are being diligent
+    # enough to type-check their 3.5-or-later code, they're probably diligent
+    # enough to pass FrameSummarys rather than make use of the
+    # only-for-backwards-compatibility four-tuple parameter type semantics.
+    def format_list(extracted_list: Iterable[FrameSummary]) -> List[str]: ...
 else:
     def extract_tb(tb: Optional[TracebackType], limit: Optional[int] = ...) -> List[_PT]: ...
     def extract_stack(f: Optional[FrameType] = ...,
@@ -90,8 +95,6 @@ if sys.version_info >= (3, 5):
         def format(self, *, chain: bool = ...) -> Generator[str, None, None]: ...
         def format_exception_only(self) -> Generator[str, None, None]: ...
 
-
-if sys.version_info >= (3, 5):
     class FrameSummary(Iterable):
         filename: str
         lineno: int
@@ -101,7 +104,7 @@ if sys.version_info >= (3, 5):
         def __init__(self, filename: str, lineno: int, name: str,
                      lookup_line: bool = ...,
                      locals: Optional[Mapping[str, str]] = ...,
-                     line: Optional[int] = ...) -> None: ...
+                     line: Optional[str] = ...) -> None: ...
         # TODO: more precise typing for __getitem__ and __iter__,
         # for a namedtuple-like view on (filename, lineno, name, str).
         def __getitem__(self, i: int) -> Any: ...