windows-ci: run Windows CI automatically alongside the others

This will enable us to make Windows Python 3 a first-class citizen for the next
6.0 cycle. We will probably get some flaky tests and we're missing others that
are skipped, but we'll turn them on it future patches.

Differential Revision: https://phab.mercurial-scm.org/D11256

contrib/heptapod-ci.yml

@@ -163,7 +163,6 b' check-pytype-py3:'
 
 windows-py3:
     <<: *windows_runtests
-    when: manual
     tags:
       - windows
     variables: