Use pypyX.Y for PyPy python-version input (#349)

This versioning scheme is consistent with other
tools in the python ecosystem so it feels more natural
and allows better interaction with other tools.

fixes #346

src/setup-python.ts

@@ -7,7 +7,7 @@ import {getCacheDistributor} from './cache-distributions/cache-factory';
 import {isCacheFeatureAvailable} from './utils';
 
 function isPyPyVersion(versionSpec: string) {
-  return versionSpec.startsWith('pypy-');
+  return versionSpec.startsWith('pypy');
 }
 
 async function cacheDependencies(cache: string, pythonVersion: string) {