Use keyword.iskeyword rather than expanding the set of reserved names

Prior to this change, if we were checking whether a non-builtin name was
allowed, we would expand the set of reserved names to include all
keywords.

This change switches to using the iskeyword function instead, which will
allow us to stop using the set of reserved names altogether in a future
commit.

Author: leamingrad

src/fluent_compiler/codegen.py

@@ -161,14 +161,14 @@ def _add(final):
 
         used = self.all_reserved_names()
 
-        # We need to also protect against using keywords ('class', 'def' etc.)
-        # i.e. count all keywords as 'used'.
-        # However, some builtins are also keywords (e.g. 'None'), and so
-        # if a builtin is being reserved, don't check against the keyword list
-        if not is_builtin:
-            used = used | set(keyword.kwlist)
-
         def _is_name_allowed(name: str) -> bool:
+            # We need to also protect against using keywords ('class', 'def' etc.)
+            # i.e. count all keywords as 'used'.
+            # However, some builtins are also keywords (e.g. 'None'), and so
+            # if a builtin is being reserved, don't check against the keyword list
+            if (not is_builtin) and keyword.iskeyword(name):
+                return False
+
             return name not in used
 
         while not _is_name_allowed(attempt):