Warn instead of failing on not_skip setting

CHANGELOG.md

@@ -12,6 +12,8 @@ NOTE: isort follows the [semver](https://semver.org/) versioning standard.
   - Fixed #1289: Stream usage no longer auto picking up config file from current working directory.
   - Fixed #1296: Force_single_line setting removes immediately following comment line.
   - Fixed #1295: `ensure_newline_before_comments` doesnt work with `force_sort_within_sections`.
+  - Setting not_skip will no longer immediately fail but instead give user a warning and direct
+    to upgrade docs.
 
 ### 5.0.4 July 6, 2020
   - Fixed #1264: a regression with comment handling and `force_sort_within_sections` config option

isort/main.py

@@ -680,7 +680,7 @@ def main(argv: Optional[Sequence[str]] = None, stdin: Optional[TextIOWrapper] =
 
     if deprecated_flags:  # pragma: no cover
         warn(
-            f"\n\nThe following deprecated CLI flags where used: {', '.join(deprecated_flags)}!\n"
+            f"\n\nThe following deprecated CLI flags were used: {', '.join(deprecated_flags)}!\n"
             "Please see the 5.0.0 upgrade guide:\n"
             "\thttps://timothycrosley.github.io/isort/docs/upgrade_guides/5.0.0/\n"
         )

isort/settings.py

@@ -62,6 +62,8 @@
 
 RUNTIME_SOURCE = "runtime"
 
+DEPRECATED_SETTINGS = ("not_skip",)
+
 
 @dataclass(frozen=True)
 class _Config:
@@ -296,6 +298,16 @@ def __init__(
         combined_config.pop("source", None)
         combined_config.pop("sources", None)
         combined_config.pop("runtime_src_paths", None)
+
+        for deprecated_option in DEPRECATED_SETTINGS:
+            if deprecated_option in combined_config:
+                warn(
+                    f"\n\nThe following deprecated settings was used: {deprecated_option}!\n"
+                    "Please see the 5.0.0 upgrade guide:\n"
+                    "\thttps://timothycrosley.github.io/isort/docs/upgrade_guides/5.0.0/\n"
+                )
+                combined_config.pop(deprecated_option)
+
         if known_other:
             for known_key in known_other:
                 combined_config.pop(f"{KNOWN_PREFIX}{known_key}", None)