Cirrus CI: Don't install pygame builddeps when a wheel is available

Dirty hack: hardcode that wheels are available except for PyPy and rc versions
of CPython.

I tried looking up that info from PyPI's API, but finding which OS a given wheel
is built for would require parsing the filename...

.cirrus.yml

@@ -23,8 +23,11 @@ Linux_task:
       - image: pypy:3.6-slim
 
   install_script:
-    - apt update || true
-    - apt install -qq -y pkgconf libsdl-image1.2-dev libsdl-mixer1.2-dev libsdl-ttf2.0-dev libsdl1.2-dev libsmpeg-dev libportmidi-dev libswscale-dev libavformat-dev libavcodec-dev libfreetype6-dev gcc
+    - >-
+      if command -v pypy3 >/dev/null || python3 -c 'import sys; exit(sys.implementation.version.releaselevel == "final")'; then
+        apt update || true;
+        apt install -qq -y pkgconf libsdl-image1.2-dev libsdl-mixer1.2-dev libsdl-ttf2.0-dev libsdl1.2-dev libsmpeg-dev libportmidi-dev libswscale-dev libavformat-dev libavcodec-dev libfreetype6-dev gcc
+      fi
     - pip install --upgrade-strategy eager -U pytest
     - pip install -e .