Bugfix: uses \UseRawInputEncoding to tell docstrip that the dtx file contains UTF8 characters.

With recent texlive (the issue was reported with TL2018), dostrip needs this command to generate the sty file properly. Without it, we got
! Undefined control sequence.
\GenericError  ...
                                                    #4  \errhelp \@err@     ...
l.54 ...}{\from{automultiplechoice.dtx}{package}}}
                                                  ^^M