Log Message:
Update to 0.21

Upstream changes:
0.21    2019-04-14 18:19:17Z
 - fix threaded builds - revert to older Perl_filter_read (RT#129173)

0.20    2019-04-14 06:54:56Z
  - now uses core functions directly, instead of code copied from core, when
    available (starting in perl 5.29.10) (Karl Williamson, pull request #3;
    see perl RT#133318)