feat: add --keep-string-format flag

Author: MrLYC

README.md

@@ -641,6 +641,7 @@ Availability:
 
 Availability:
 - `--py36-plus` is passed on the commandline.
+- Unless `--keep-string-format` is passed.
 
 ```diff
 -'{foo} {bar}'.format(foo=foo, bar=bar)

pyupgrade/_data.py

@@ -22,6 +22,7 @@ class Settings(NamedTuple):
     keep_percent_format: bool = False
     keep_mock: bool = False
     keep_runtime_typing: bool = False
+    keep_string_format: bool = False
 
 
 class State(NamedTuple):

pyupgrade/_main.py

@@ -322,6 +322,7 @@ def _fix_file(filename: str, args: argparse.Namespace) -> int:
             keep_percent_format=args.keep_percent_format,
             keep_mock=args.keep_mock,
             keep_runtime_typing=args.keep_runtime_typing,
+            keep_string_format=args.keep_string_format,
         ),
     )
     contents_text = _fix_tokens(contents_text)
@@ -346,6 +347,7 @@ def main(argv: Sequence[str] | None = None) -> int:
     parser.add_argument('--keep-percent-format', action='store_true')
     parser.add_argument('--keep-mock', action='store_true')
     parser.add_argument('--keep-runtime-typing', action='store_true')
+    parser.add_argument('--keep-string-format', action='store_true')
     parser.add_argument(
         '--py3-plus', '--py3-only',
         action='store_const', dest='min_version', default=(3,), const=(3,),

pyupgrade/_plugins/fstrings.py

@@ -134,7 +134,8 @@ def visit_Call(
                     i += 1
         else:
             if (
-                    state.settings.min_version >= (3, 7) or
-                    not contains_await(node)
+                    not state.settings.keep_string_format and
+                    (state.settings.min_version >= (3, 7) or
+                     not contains_await(node))
             ):
                 yield ast_to_offset(node), _fix_fstring

tests/features/fstrings_test.py

@@ -79,3 +79,11 @@ def test_fix_fstrings_await_py37():
     s = "async def c(): return '{}'.format(await 1+foo())"
     expected = "async def c(): return f'{await 1+foo()}'"
     assert _fix_plugins(s, settings=Settings(min_version=(3, 7))) == expected
+
+
+def test_keep_string_format():
+    s = '"{} {}".format(a, b)'
+    assert _fix_plugins(
+        s, 
+        settings=Settings(min_version=(3, 6), keep_string_format=True),
+    ) == s