Log Message:
Update to 0.9724

Upstream changes:
0.9724 2021-09-13
- make deep_copy not interfere with $. - thanks @merkys for report

0.9723 2021-09-01
- doc fixes - thanks @xsawyerx
- fix problem with deep_copy with vertices that are refs - thanks @merkys for report

0.9722 2021-07-04
- fix neighbours et al not returning count in scalar context - thanks @merkys \ 
for report