Add missing include

[obackhouse]

Installation using pip fails as

      In file included from gristmill/_parenth.cpp:13:
      /tmp/pip-install-iwdh__o7/gristmill_8be63384772f4f05b78c42c9c06934b7/deps/cpypp/include/cpypp.hpp:726:29: error: ‘ptrdiff_t’ does not name a type
        726 |     using difference_type = ptrdiff_t;
            |                             ^~~~~~~~~
      /tmp/pip-install-iwdh__o7/gristmill_8be63384772f4f05b78c42c9c06934b7/deps/cpypp/include/cpypp.hpp:17:1: note: ‘ptrdiff_t’ is defined in header ‘<cstddef>’; did you forget to ‘#include <cstddef>’?
         16 | #include <cassert>
        +++ |+#include <cstddef>
         17 | #include <utility>

[chenpeizhi]

Thank you! Sorry for the delayed merge.

[obackhouse]

Thank you! Sorry for the delayed merge.

no worries, i have some more for drudge that i've submitted (DrudgeCAS/drudge#28) -- i never bothered since there was no response here so i was just working out of my forks.

Also, DummySpark is broken because Iterable is now in collections.abc rather than collections. I am happy to fix.

[chenpeizhi]

Thanks. As you mentioned in your latest PR, we have recently started to clean up the Drudge/Gristmill codebase and have fixed a few compatibility issues. Please keep us posted if you notice any other issues. PRs are also welcome.