Backport PR #4181: nbconvert: Fix, sphinx template not removing new lines from headers

If 1.1 hasn't been released yet, it would be nice to backport this.

See  #4122

IPython/nbconvert/templates/latex/sphinx.tplx

@@ -229,7 +229,7 b' Note: For best display, use latex syntax highlighting. =))'
     in IPYNB file titles) do not make their way into latex.  Sometimes this
     causes latex to barf. =))
     ((*- endif -*))
-    {((( cell.source | markdown2latex )))}
+    {((( cell.source | replace('\n', ' ') | markdown2latex )))}
 ((*- endblock headingcell *))
 
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