add (overwrite with) mypy stubs, if available

stdlib/3/string.pyi

@@ -0,0 +1,27 @@
+# Stubs for string
+
+# Based on http://docs.python.org/3.2/library/string.html
+
+from typing import Mapping
+
+ascii_letters = ''
+ascii_lowercase = ''
+ascii_uppercase = ''
+digits = ''
+hexdigits = ''
+octdigits = ''
+punctuation = ''
+printable = ''
+whitespace = ''
+
+def capwords(s: str, sep: str = None) -> str: ...
+
+class Template:
+    template = ''
+
+    def __init__(self, template: str) -> None: ...
+    def substitute(self, mapping: Mapping[str, str], **kwds: str) -> str: ...
+    def safe_substitute(self, mapping: Mapping[str, str],
+                        **kwds: str) -> str: ...
+
+# TODO Formatter