Do not match string prefix at end of docstring

The original docstring regex removed sequences such as `r"""` from a
docstring regardless if it appeared at the beginning or at the end of
the docstring: `r"""Simulator"""` erroneously becomes `Simulato`.
However, `r` etc. are
[string prefixes](https://docs.python.org/3/reference/lexical_analysis.html#string-and-bytes-literals)
and do not appear at the end of a string.

Therefore, this commit changes the regex to match string prefixes only
at the beginning of the docstring and not at the end.

(Closes Feneric#84)

doxypypy/doxypypy.py

@@ -56,7 +56,8 @@ class AstWalker(NodeVisitor):
     __indentRE = regexpCompile(r'^(\s*)\S')
     __newlineRE = regexpCompile(r'^#', MULTILINE)
     __blanklineRE = regexpCompile(r'^\s*$')
-    __docstrMarkerRE = regexpCompile(r"\s*([uUbB]*[rR]?(['\"]{3}))")
+    __docstrMarkerRE = regexpCompile(r"^\s*([uUbB]*[rR]?(['\"]{3}))|"
+                                     r"\s*(['\"]{3})\s*$")
     __docstrOneLineRE = regexpCompile(r"\s*[uUbB]*[rR]?(['\"]{3})(.+)\1")
 
     __implementsRE = regexpCompile(r"^(\s*)(?:zope\.)?(?:interface\.)?"