Update conf.py to execute imports during pdf building

[gepcel]

closes #38451

According https://ipython.readthedocs.io/en/stable/sphinxext.html, it should be ipython_execlines not ipython_exec_lines. Should close issue #38451

[jreback]

hmm so it is.

[jreback]

cc @simonjayhawkins

[simonjayhawkins]

Thanks @gepcel for the PR. will check pdf doc build shortly on my release environment for 1.2 with this change

changed milestone as we will want to backport this, if merged.

[jreback]

@simonjayhawkins how's this looking

[simonjayhawkins]

will merge this to close #38451 as it does prevent the NameError.

However, something else is awry since {{ header }} still appears in the pdf docs.

During the first run of ./make.py latex_forced it appears in 10 minutes to pandas and no contents are created. we run a second time (I assume to get the contents to build) and on the second run the visualisaztion section is recreated (why this happens is they key to solving this properly) but without the header code being run but the change here means that we don't get the NameErrors

[simonjayhawkins]

Thanks @gepcel

[gepcel]

However, something else is awry since {{ header }} still appears in the pdf docs.

That might because the section (reference) name is "visualization" and the section title is also "visualization", therefore the Duplicate implicit target name: "visualization".