py/modsys: Rename sys.implementation.mpy to sys.implementation._mpy.

Per CPython docs, non-standard attributes must begin with an underscore.

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

docs/library/sys.rst

@@ -70,6 +70,7 @@ Constants
 
    * *name* - string "micropython"
    * *version* - tuple (major, minor, micro), e.g. (1, 7, 0)
+   * *_mpy* - supported mpy file-format version (optional attribute)
 
    This object is the recommended way to distinguish MicroPython from other
    Python implementations (note that it still may not exist in the very

docs/reference/mpyfiles.rst

@@ -44,7 +44,7 @@ Compatibility is based on the following:
   loading it must support execution of that architecture's code.
 
 If a MicroPython system supports importing .mpy files then the
-``sys.implementation.mpy`` field will exist and return an integer which
+``sys.implementation._mpy`` field will exist and return an integer which
 encodes the version (lower 8 bits), features and native architecture.
 
 Trying to import an .mpy file that fails one of the first four tests will
@@ -58,7 +58,7 @@ If importing an .mpy file fails then try the following:
   by executing::
 
     import sys
-    sys_mpy = sys.implementation.mpy
+    sys_mpy = sys.implementation._mpy
     arch = [None, 'x86', 'x64',
         'armv6', 'armv6m', 'armv7m', 'armv7em', 'armv7emsp', 'armv7emdp',
         'xtensa', 'xtensawin'][sys_mpy >> 10]

py/modsys.c

@@ -84,7 +84,7 @@ STATIC const qstr impl_fields[] = {
     MP_QSTR_name,
     MP_QSTR_version,
     #if MICROPY_PERSISTENT_CODE_LOAD
-    MP_QSTR_mpy,
+    MP_QSTR__mpy,
     #endif
 };
 STATIC MP_DEFINE_ATTRTUPLE(