It looks like the 3.6 and 3.7 full doc build has been failing since 2017-10-15.  Seen in /var/log/docsbuild/python37-en.log:

[...]
[3] [4] [5] [6] [7] (/usr/share/texlive/texmf-dist/tex/latex/ucs/data/uni-1.def
) (/usr/share/texlive/texmf-dist/tex/latex/ucs/data/uninames.dat)
(/usr/share/texlive/texmf-dist/tex/latex/ucs/data/uni-1.def)

! Package ucs Error: Unknown Unicode character 383 = U+017F,
(ucs)                possibly declared in uni-1.def.
(ucs)                Type H to see if it is available with options.

See the ucs package documentation for explanation.
Type  H <return>  for immediate help.
 ...                                              
                                                  
l.781 Latin small letter dotless i), ‘ſ
                                          ’ (U+017F, Latin small letter lo...

? 
! Emergency stop.
 ...                                              
                                                  
l.781 Latin small letter dotless i), ‘ſ
                                          ’ (U+017F, Latin small letter lo...

!  ==> Fatal error occurred, no output PDF file produced!
Transcript written on howto-regex.log.
Latexmk: Index file 'howto-regex.idx' was written
Collected error summary (may duplicate other messages):
  pdflatex: Command for 'pdflatex' gave return code 256
Latexmk: Use the -f option to force complete processing,
 unless error was exceeding maximum runs of latex/pdflatex.
Latexmk: Errors, so I did not complete making targets
make[2]: *** [howto-regex.pdf] Error 12
make[2]: Leaving directory `/srv/docsbuild/python37/Doc/build/latex'
make[1]: *** [dist] Error 2
make[1]: Leaving directory `/srv/docsbuild/python37/Doc'
make: *** [autobuild-dev] Error 2

Julien, can you take a look please?