refactor: update _LockFileEncoder to Python3 syntax

Also, added the encode method in addition to the iterencode
method.
This way we can later import this class in Pipenv instead
of redefining it there.

src/plette/lockfiles.py

@@ -13,27 +13,36 @@ class _LockFileEncoder(json.JSONEncoder):
     This adds a few characteristics to the encoder:
 
     * The JSON is always prettified with indents and spaces.
-    * The output is always UTF-8-encoded text, never binary, even on Python 2.
+    * TOMLKit's container elements are seamlessly encodable.
+    * The output is always UTF-8-encoded text, never binary.
     """
     def __init__(self):
-        super(_LockFileEncoder, self).__init__(
-            indent=4, separators=(",", ": "), sort_keys=True,
+        super().__init__(
+            indent=4,
+            separators=(",", ": "),
+            sort_keys=True,
         )
 
     def encode(self, obj):
-        content = super(_LockFileEncoder, self).encode(obj)
+        content = super().encode(obj)
         if not isinstance(content, str):
             content = content.decode("utf-8")
         content += "\n"
         return content
 
     def iterencode(self, obj):
-        for chunk in super(_LockFileEncoder, self).iterencode(obj):
+        for chunk in super().iterencode(obj):
             if not isinstance(chunk, str):
                 chunk = chunk.decode("utf-8")
             yield chunk
         yield "\n"
 
+    def encode(self, obj):
+        content = super().encode(obj)
+        if not isinstance(content, str):
+            content = content.decode("utf-8")
+        return content
+
 
 PIPFILE_SPEC_CURRENT = 6