py/modsys: Allow sys.path to be assigned to.

Previously sys.path could be modified by append/pop or slice assignment.

This allows `sys.path = [...]`, which can be simpler in many cases, but
also improves CPython compatibility.

It also allows sys.path to be set to a tuple which means that you can
clear sys.path (e.g. temporarily) with no allocations.

This also makes sys.path (and sys.argv for consistency) able to be disabled
via mpconfig. The unix port (and upytesthelper) require them, so they
explicitly verify that they're enabled.

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

py/mpstate.h

@@ -43,6 +43,9 @@
 #if MICROPY_PY_SYS_ATTR_DELEGATION
 // Must be kept in sync with sys_mutable_keys in modsys.c.
 enum {
+    #if MICROPY_PY_SYS_PATH
+    MP_SYS_MUTABLE_PATH,
+    #endif
     #if MICROPY_PY_SYS_PS1_PS2
     MP_SYS_MUTABLE_PS1,
     MP_SYS_MUTABLE_PS2,