New in version 1.1.1.
Return Abjad centered page number Markup.
abjad> t = markup.centered_page_number( )
abjad> print t.format
\markup {
\fill-line {
\bold \fontsize #3
\on-the-fly #print-page-number-check-first
\fromproperty #'page:page-number-string } }