Add zerver/lib/str_utils.py.

str_utils.py has functions for converting strings from one type to
another.  It also has a TypeVar called NonBinaryStr, which is like AnyStr
except that it doesn't allow bytes.

zerver/lib/str_utils.py

@@ -0,0 +1,40 @@
+import six
+from six import text_type, binary_type
+from typing import Any, Mapping, Union, TypeVar
+
+NonBinaryStr = TypeVar('NonBinaryStr', str, text_type)
+# This is used to represent text or native strings
+
+def force_text(s):
+    # type: (Union[text_type, binary_type]) -> text_type
+    if isinstance(s, text_type):
+        return s
+    elif isinstance(s, binary_type):
+        return s.decode('utf-8')
+    else:
+        raise ValueError("force_text expects a string type")
+
+def force_bytes(s):
+    # type: (Union[text_type, binary_type]) -> binary_type
+    if isinstance(s, binary_type):
+        return s
+    elif isinstance(s, text_type):
+        return s.encode('utf-8')
+    else:
+        raise ValueError("force_bytes expects a string type")
+
+def force_str(s):
+    # type: (Union[text_type, binary_type]) -> str
+    if isinstance(s, str):
+        return s
+    elif isinstance(s, text_type):
+        return s.encode('utf-8')
+    elif isinstance(s, binary_type):
+        return s.decode('utf-8')
+    else:
+        raise ValueError("force_str expects a string type")
+
+def dict_with_str_keys(dct):
+    # type: (Mapping[NonBinaryStr, Any]) -> Dict[str, Any]
+    """applies force_str on the keys of a dict (non-recursively)"""
+    return {force_str(key): value for key, value in six.iteritems(dct)}