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BUG27837814: Fix missing .pyc files in Python 3 commercial packages

Add pip installation instructions in the docs

BUG26847553: Fix AttributeError when using pure Python Protobuf

Using a value of type list when setting an attribute in a Message object
throws an AttributeError.

This patch fix this issue by testing the value type and using the .extend()
method for list type values.

This patch also adds the encoding of string values types to bytes when using
Python 3.

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Add Python 3.4 and 3.5 as supported versions

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BUG25215081: Remove dictionary comprehension lists

Dictionary comprehension lists is not supported in python 2.6 which causes
this error. Using the dict initializer with tuple works the same as
dictionary comprehension lists and is also supported in python 2.6.

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