Fix building spidermonkey on systems with python 3.10
Tested by:
- Langbart - macOS 10.15.7: homebrewed python 3.9.9 & 3.10.1
- andy5995 - Manjaro 21.2.3: python 3.10.2
- s0600204 - ArchLinux: python 3.10.2
Differential Revision: https://code.wildfiregames.com/D4437