Replace default strings ' ' with ... (#1721)

This fixes a problem with pytype.

Author: x3k6a2

stdlib/3/builtins.pyi

@@ -238,7 +238,7 @@ class str(Sequence[str]):
 
     def capitalize(self) -> str: ...
     def casefold(self) -> str: ...
-    def center(self, width: int, fillchar: str = ' ') -> str: ...
+    def center(self, width: int, fillchar: str = ...) -> str: ...
     def count(self, x: str, __start: Optional[int] = ..., __end: Optional[int] = ...) -> int: ...
     def encode(self, encoding: str = 'utf-8', errors: str = 'strict') -> bytes: ...
     def endswith(self, suffix: Union[str, Tuple[str, ...]], start: Optional[int] = None,
@@ -260,14 +260,14 @@ class str(Sequence[str]):
     def istitle(self) -> bool: ...
     def isupper(self) -> bool: ...
     def join(self, iterable: Iterable[str]) -> str: ...
-    def ljust(self, width: int, fillchar: str = ' ') -> str: ...
+    def ljust(self, width: int, fillchar: str = ...) -> str: ...
     def lower(self) -> str: ...
     def lstrip(self, chars: Optional[str] = None) -> str: ...
     def partition(self, sep: str) -> Tuple[str, str, str]: ...
     def replace(self, old: str, new: str, count: int = -1) -> str: ...
     def rfind(self, sub: str, __start: Optional[int] = ..., __end: Optional[int] = ...) -> int: ...
     def rindex(self, sub: str, __start: Optional[int] = ..., __end: Optional[int] = ...) -> int: ...
-    def rjust(self, width: int, fillchar: str = ' ') -> str: ...
+    def rjust(self, width: int, fillchar: str = ...) -> str: ...
     def rpartition(self, sep: str) -> Tuple[str, str, str]: ...
     def rsplit(self, sep: Optional[str] = None, maxsplit: int = -1) -> List[str]: ...
     def rstrip(self, chars: Optional[str] = None) -> str: ...
@@ -782,7 +782,7 @@ def filter(function: Optional[Callable[[_T], Any]],
            iterable: Iterable[_T]) -> Iterator[_T]: ...
 @overload
 def filter(function: None, iterable: Iterable[Optional[_T]]) -> Iterator[_T]: ...
-def format(o: object, format_spec: str = '') -> str: ...
+def format(o: object, format_spec: str = ...) -> str: ...
 def getattr(o: Any, name: str, default: Any = ...) -> Any: ...
 def globals() -> Dict[str, Any]: ...
 def hasattr(o: Any, name: str) -> bool: ...
@@ -834,7 +834,7 @@ else:
 
 def ord(c: Union[str, bytes, bytearray]) -> int: ...
 # TODO: in Python 3.2, print() does not support flush
-def print(*values: Any, sep: str = ' ', end: str = '\n', file: Optional[IO[str]] = None, flush: bool = False) -> None: ...
+def print(*values: Any, sep: str = ..., end: str = ..., file: Optional[IO[str]] = None, flush: bool = False) -> None: ...
 @overload
 def pow(x: int, y: int) -> Any: ...  # The return type can be int or float, depending on y
 @overload