Import Upstream version 2.7.18

Demo/classes/README

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+Examples of classes that implement special operators (see reference manual):
+
+Complex.py	Complex numbers
+Dates.py	Date manipulation package by Tim Peters
+Dbm.py		Wrapper around built-in dbm, supporting	arbitrary values
+Range.py	Example of a generator: re-implement built-in range()
+Rev.py		Yield the reverse of a sequence
+Vec.py		A simple vector class
+bitvec.py	A bit-vector class by Jan-Hein B\"uhrman
+
+(For straightforward examples of basic class features, such as use of
+methods and inheritance, see the library code.)