This should fix this error which appears on RHEL only:
```
Configuration error:
There is a programmable error in your configuration file:
Traceback (most recent call last):
File "/usr/lib/python3.9/site-packages/sphinx/config.py", line 326, in eval_config_file
execfile_(filename, namespace)
File "/usr/lib/python3.9/site-packages/sphinx/util/pycompat.py", line 88, in execfile_
exec(code, _globals)
File "/builddir/build/BUILD/llvm-project-c1d5cc99c6ba8e897ea145dbb2221a155b5e3e5a/llvm/redhat-linux-build/tools/clang/docs/conf.py", line 42, in <module>
import myst_parser
ModuleNotFoundError: No module named 'myst_parser'
```
See also https://github.com/fedora-llvm-team/llvm-snapshots/issues/492
