Running make in the Doc directory does not install all necessary tooling on its own.  This is a hurdle to getting beginners up to speed on making documentation changes (often their very first changes in the CPython project).

```
:~/oss/cpython/throwaway:master$ make -C Doc html
make: Entering directory '/.../oss/cpython/throwaway/Doc'
mkdir -p build
Building NEWS from Misc/NEWS.d with blurb
/bin/sh: line 5: blurb: command not found
Makefile:44: recipe for target 'build' failed
make: *** [build] Error 127
make: Leaving directory '/.../oss/cpython/throwaway/Doc'
```

Doc builders need to run `make -C Doc venv` first.  Just updating the logic to print that as an error message when the venv or any of the necessary tools within it are missing would be more welcoming.