Mention that building Python docs now requires blurb

[Mariatta]

This section describes how to build the docs: https://devguide.python.org/documenting/#building-the-documentation

It currently mentions that we need to have sphinx installed. However, I noticed that blurb is now needed for building the docs.

make html will fail if we don't have blurb installed.

The documentation should be updated with the instruction to install blurb too.

[matrixise]

I have read the code of Doc/Makefile, and we install blurb when we create the venv, https://github.com/python/cpython/blob/master/Doc/Makefile#L126

Do we need to update the documentation ?

[Mariatta]

Hmm maybe something wrong with my computer setup?

I did make html from the Doc directory and it does not work. Here's the output from my terminal:

$ make -C Doc html
mkdir -p build
Building NEWS from Misc/NEWS.d with blurb
/bin/sh: blurb: command not found
make: *** [build] Error 127
$ cd Doc
$ make html
mkdir -p build
Building NEWS from Misc/NEWS.d with blurb
/bin/sh: blurb: command not found
make: *** [build] Error 127
$

Once I manually created the venv and installed sphinx and blurb, then it works.

[Mariatta]

The paragraph says: "You are expected to have installed the latest stable version of Sphinx on your system or in a virtualenv, so that the Makefile can find the sphinx-build command. You can also specify the location of sphinx-build with the SPHINXBUILD make variable."

So it should mention about adding blurb in addition to sphinx, I think.

[bukun]

blurb could be installed via pip.

pip3 install blurb