[Python] fix and improve default argument handling

1. Fix negative octals. Currently not handled correctly by `-py3`
   (unusual case, but incorrect).
2. Fix arguments of type "octal + something" (e.g. `0640 | 04`).
   Currently drops everything after the first octal. Nasty!
3. Fix bool arguments "0 + something" (e.g. `0 | 1`) are always
   "False" (unusual case, but incorrect).
4. Remove special handling of "TRUE" and "FALSE" from
   `convertValue` since there's no reason these have to match
   "true" and "false".
5. Remove the Python 2 vs. Python 3 distinction based on the
   `-py3` flag. Now the same python code is produced for default
   arguments for Python 2 and Python 3. For this, octal default
   arguments, e.g. 0644, are now wrapped as `int('644', 8)`. This
   is required, as Python 2 and Python 3 have incompatible syntax
   for octal literals.

Fixes #707

Examples/test-suite/default_args.i

@@ -18,14 +18,20 @@
   #include <string>
 
   // All kinds of numbers: hex, octal (which pose special problems to Python), negative...
-  void trickyvalue1(int first, int pos = -1) {}
-  void trickyvalue2(int first, unsigned rgb = 0xabcdef) {}
-  void trickyvalue3(int first, int mode = 0644) {}
+
+  class TrickyInPython {
+  public:
+    int value_m1(int first, int pos = -1) { return pos; }
+    unsigned value_0xabcdef(int first, unsigned rgb = 0xabcdef) { return rgb; }
+    int value_0644(int first, int mode = 0644) { return mode; }
+    int value_perm(int first, int mode = 0640 | 0004) { return mode; }
+    int value_m01(int first, int val = -01) { return val; }
+    bool booltest2(bool x = 0 | 1) { return x; }
+  };
 
   void doublevalue1(int first, double num = 0.0e-1) {}
   void doublevalue2(int first, double num = -0.0E2) {}
 
-  // Long long arguments are not handled at Python level currently but still work.
   void seek(long long offset = 0LL) {}
   void seek2(unsigned long long offset = 0ULL) {}
   void seek3(long offset = 0L) {}