Ensure that _vim_{dec,enc} always succeeds on py3

In py2 we could get by with attempting to encode/decode a string and if
it failed just ignore and return.  Unfortunately on py3 a string decodes
to a bytearray and a bytearray encodes to a string.  We can't simply
return the original object when we fail.  We choose replace for handling
bytes we don't understand.

Test case (using python3 for UltiSnips):
open a new file
:set encoding=utf-8
qa<Backspace>q
"ap

Warning: this test case makes it very difficult to exit vim cleanly and
save files!

Signed-off-by: Andrew Ruder <andrew.ruder@elecsyscorp.com>

doc/UltiSnips.txt

@@ -1384,6 +1384,7 @@ individuals have contributed to UltiSnips (in chronological order):
    Pedro Ferrari - petobens
    Ches Martin - ches
    Christian - Oberon00
+   Andrew Ruder - aeruder
 
 Thank you for your support.
 

pythonx/UltiSnips/compatibility.py

@@ -10,22 +10,21 @@ import sys
 
 import vim  # pylint:disable=import-error
 
+if sys.version_info >= (3, 0):
     def _vim_dec(string):
         """Decode 'string' using &encoding."""
-    try:
-        return string.decode(vim.eval("&encoding"))
-    except UnicodeDecodeError:
-        # At least we tried. There might be some problems down the road now
-        return string
+        # We don't have the luxury here of failing, everything
+        # falls apart if we don't return a bytearray from the
+        # passed in string
+        return string.decode(vim.eval("&encoding"), "replace")
 
-def _vim_enc(string):
+    def _vim_enc(bytearray):
         """Encode 'string' using &encoding."""
-    try:
-        return string.encode(vim.eval("&encoding"))
-    except UnicodeEncodeError:
-        return string
+        # We don't have the luxury here of failing, everything
+        # falls apart if we don't return a string from the passed
+        # in bytearray
+        return bytearray.encode(vim.eval("&encoding"), "replace")
 
-if sys.version_info >= (3, 0):
     def open_ascii_file(filename, mode):
         """Opens a file in "r" mode."""
         return open(filename, mode, encoding="utf-8")
@@ -60,6 +59,22 @@ if sys.version_info >= (3, 0):
 else:
     import warnings
     warnings.filterwarnings("ignore", category=DeprecationWarning)
+
+    def _vim_dec(string):
+        """Decode 'string' using &encoding."""
+        try:
+            return string.decode(vim.eval("&encoding"))
+        except UnicodeDecodeError:
+            # At least we tried. There might be some problems down the road now
+            return string
+
+    def _vim_enc(string):
+        """Encode 'string' using &encoding."""
+        try:
+            return string.encode(vim.eval("&encoding"))
+        except UnicodeEncodeError:
+            return string
+
     def open_ascii_file(filename, mode):
         """Opens a file in "r" mode."""
         return open(filename, mode)