use fantastic f-strings

Author: MarcoGorelli

pandas/_libs/tslibs/strptime.pyx

@@ -59,16 +59,13 @@ cdef bint format_is_iso(f: str):
     Generally of form YYYY-MM-DDTHH:MM:SS - date separator can be different
     but must be consistent.  Leading 0s in dates and times are optional.
     """
-    iso_template = "%Y{date_sep}%m{date_sep}%d{time_sep}%H:%M:%S{micro_or_tz}".format
     excluded_formats = ["%Y%m"]
 
     for date_sep in [" ", "/", "\\", "-", ".", ""]:
         for time_sep in [" ", "T"]:
             for micro_or_tz in ["", "%z", ".%f", ".%f%z"]:
-                if (iso_template(date_sep=date_sep,
-                                 time_sep=time_sep,
-                                 micro_or_tz=micro_or_tz,
-                                 ).startswith(f) and f not in excluded_formats):
+                iso_fmt = f"%Y{date_sep}%m{date_sep}%d{time_sep}%H:%M:%S{micro_or_tz}"
+                if iso_fmt.startswith(f) and f not in excluded_formats:
                     return True
     return False