python3: More of 'prefer unix sep if MSYSTEM'

os.getcwd and _getfullpathname

Also stopped using 0590-mingw-INSTALL_SHARED-LDLIBRARY-LIBPL.patch
as I'm fairly sure it's not necessary.

mingw-w64-python3/0810-remove_path_max.default.patch

@@ -2,9 +2,9 @@ diff -urN a/Include/osdefs.h b/Include/osdefs.h
 --- a/Include/osdefs.h	2014-10-11 14:23:05.655352800 +0100
 +++ b/Include/osdefs.h	2014-10-11 14:23:07.572462400 +0100
 @@ -17,7 +17,6 @@
+ #if defined(MS_WINDOWS) || defined(__BORLANDC__) || defined(__WATCOMC__) || defined(__DJGPP__)
  #define SEP L'\\'
  #define ALTSEP L'/'
- #endif
 -#define MAXPATHLEN 256
  #define DELIM L';'
  #define DELIMSTR ";"