Simplify test_ident using str.isidentifier()

I have checked that this gives the same results as the previous method.

tools/gen_latex_symbols.py

@@ -46,13 +46,10 b' lines = [line_to_tuple(line) for line in lines]'
 print('Filtering out characters that are not valid Python 3 identifiers')
 
 def test_ident(i):
-    """Is the unicode string a valid Python 3 identifer."""
-    try:
-        exec('a%s = 10' % i, {}, {})
-    except SyntaxError:
-        return False
-    else:
-        return True
+    """Is the unicode string valid in a Python 3 identifer."""
+    # Some characters are not valid at the start of a name, but we still want to
+    # include them. So prefix with 'a', which is valid at the start.
+    return ('a' + i).isidentifier()
 
 assert test_ident("α")
 assert not test_ident('‴')