py/modsys: Add optional mutable attributes sys.ps1/ps2 and use them.

This allows customising the REPL prompt strings.

Signed-off-by: Damien George <damien@micropython.org>

tests/unix/extra_coverage.py.exp

@@ -45,8 +45,8 @@ utime           utimeq
 argv            atexit          byteorder       exc_info
 exit            getsizeof       implementation  maxsize
 modules         path            platform        print_exception
-stderr          stdin           stdout          tracebacklimit
-version         version_info
+ps1             ps2             stderr          stdin
+stdout          tracebacklimit  version         version_info
 ementation
 # attrtuple
 (start=1, stop=2, step=3)