Dropping Python 2 drops pypy3 from github workflows

[mgedmin]

Steps to reproduce:

1. git clone https://github.com/mgedmin/irclog2html
2. git checkout 52c154033586a31961a172efc315218e8bff7cd4
3. check-python-versions --drop 2.7

What happens:

--- ./.github/workflows/build.yml	(original)
+++ ./.github/workflows/build.yml	(updated)
@@ -20,13 +20,10 @@
     strategy:
       matrix:
         python-version:
-          - "2.7"
           - "3.6"
           - "3.7"
           - "3.8"
           - "3.9"
-          - "pypy2"
-          - "pypy3"
 
     steps:
       - name: Git clone

What should happen: basically the same, except pypy3 should remain in place.

[mgedmin]

This bug affected all workflow files using strategy.matrix.python-version, but didn't affect workflows using ZopeFoundation-style strategy.matrix.config.