build: use 'jupyter nbconvert' instead of 'ipython nbconvert'

The latter is deprecatd.

* configure.ac: Check for jupyter.
* tests/Makefile.am: Adjust.
* HACKING: Mention we need jupyter.

HACKING

@@ -43,6 +43,7 @@ since the generated files they produce are distributed.)
     like dsfont.sty.
   ImageMagick
   Python >= 3.3, IPython >= 2.3
+  Jupyter >= 4, with nbconvert
   GraphViz
   Java >= 1.7 (needed to run PlantUML while generating the doc)
   wget or curl (needed to download PlantUML)

configure.ac

@@ -197,6 +197,7 @@ AC_CHECK_PROG([SPIN], [spin], [spin])
 AC_CHECK_PROG([LBTT], [lbtt], [lbtt])
 AC_CHECK_PROG([EMACS], [emacs], [emacs])
 AC_CHECK_PROGS([IPYTHON], [ipython3 ipython], [ipython])
+AC_CHECK_PROGS([JUPYTER], [jupyter], [jupyter])
 AC_CHECK_PROG([LBTT_TRANSLATE], [lbtt-translate], [lbtt-translate])
 AX_CHECK_VALGRIND
 # Debian used to reserve the name 'swig' for swig-2.0.  So prefer

tests/Makefile.am

@@ -457,7 +457,7 @@ CLEANFILES = python/test1.dve python/test1.dve2C python/test1.dve.cpp
 
 SUFFIXES = .ipynb .html
 .ipynb.html:
-	$(IPYTHON) nbconvert $< --to html --stdout >$@
+	$(JUPYTER) nbconvert $< --to html --stdout >$@
 
 .PHONY: nb-html
 nb-html: $(TESTS_ipython:.ipynb=.html)