Revert "checked for str"

This reverts commit b63aef2.

Author: debnathshoham

pandas/io/parsers/base_parser.py

@@ -532,11 +532,7 @@ def _convert_to_ndarrays(
             conv_f = None if converters is None else converters.get(c, None)
             if isinstance(dtypes, dict):
                 cast_type = dtypes.get(c, None)
-                if (
-                    cast_type is None
-                    and isinstance(c, str)
-                    and c.split(".")[-1].isnumeric()
-                ):
+                if cast_type is None and c.split(".")[-1].isnumeric:
                     orig_c = ".".join(c.split(".")[:-1])
                     cast_type = dtypes.get(orig_c, None)
             else: