Add pre and postinstall scripts to Python component package to prevent active Python from being removed during an upgrade

code/tools/pkgresources/Scripts_python/postinstall

@@ -1,4 +1,4 @@
 #!/bin/sh
 
-# currently does nothing at all
-exit 0
+# undo the removal prevention done in preinstall
+/usr/bin/chflags -fR noschg,nouchg /usr/local/munki/Python.framework/Versions/3.*

code/tools/pkgresources/Scripts_python/preinstall

@@ -0,0 +1,8 @@
+#!/bin/sh
+
+# we're going to install version 3.10
+# prevent installer from removing a version currently in use
+# (must update this script when we move to a new Python version)
+for OLDVERS in 3.7 3.8 3.9 ; do
+    /usr/bin/chflags -fR schg,uchg /usr/local/munki/Python.framework/Versions/$OLDVERS
+done