Make Source explicitly implement __iter__()

Source was previously iterable because it implements `__getitem__()`,
which is apparently a thing from before `__iter__()` was introduced.
To reduce mypy's and my own confusion, implement `__iter__()` directly.

src/_pytest/_code/source.py

@@ -8,6 +8,7 @@ import warnings
 from ast import PyCF_ONLY_AST as _AST_FLAG
 from bisect import bisect_right
 from types import FrameType
+from typing import Iterator
 from typing import List
 from typing import Optional
 from typing import Sequence
@@ -73,6 +74,9 @@ class Source:
             newsource.lines = self.lines[key.start : key.stop]
             return newsource
 
+    def __iter__(self) -> Iterator[str]:
+        return iter(self.lines)
+
     def __len__(self) -> int:
         return len(self.lines)