update for 3.7 compat

Author: a-r-j

biopandas/pdb/pandas_pdb.py

@@ -312,7 +312,8 @@ def _parse_header_code(self):
             header = self.df["OTHERS"][self.df["OTHERS"]["record_name"] == "HEADER"]
             if not header.empty:
                 header = header["entry"].values[0]
-                if s := header.split():
+                s = header.split()
+                if s:
                     code = s[-1].lower()
         return header, code