Move site-packages to /lib/pypyX.X/site-packages, so it matches Python’s
behavior.

diff -Naur pypy3.6-v7.3.1-src.orig/lib-python/3/distutils/command/install.py pypy3.6-v7.3.1-src/lib-python/3/distutils/command/install.py
--- pypy3.6-v7.3.1-src.orig/lib-python/3/distutils/command/install.py	1970-01-01 01:00:01.000000000 +0100
+++ pypy3.6-v7.3.1-src/lib-python/3/distutils/command/install.py	2020-07-11 10:18:34.357208155 +0200
@@ -44,9 +44,9 @@
         },
     'nt': WINDOWS_SCHEME,
     'pypy': {
-        'purelib': '$base/site-packages',
-        'platlib': '$base/site-packages',
-        'headers': '$base/include/$dist_name',
+        'purelib': '$base/lib/pypy$py_version_short/site-packages',
+        'platlib': '$platbase/lib/pypy$py_version_short/site-packages',
+        'headers': '$base/include/pypy$py_version_short$abiflags/$dist_name',
         'scripts': '$base/bin',
         'data'   : '$base',
         },