Fix bug in docstring triple quotes

Problem was not escaping """ inside """.
Use ''' when possible; and when not, use: \"\"\".

test/simple_source/stmts/00_docstring.py

@@ -0,0 +1,7 @@
+# uncompyle2 bug was not escaping """ properly
+r'''func placeholder - with ("""\nstring\n""")'''
+def foo():
+    r'''func placeholder - ' and with ("""\nstring\n""")'''
+
+def bar():
+    r"""func placeholder - ' and with ('''\nstring\n''') and \"\"\"\nstring\n\"\"\" """