finish inherit chapter section 1,2

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add update

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finish inheritance chapter section 3-6

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finish inheritance section 7

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finish inheritance section 8,9

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Merge branch 'master' of https://github.com/carfly/thinkpython-cn
finish inheritance section 10

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finish inheritance section 11

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doing inheritance section 12

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finish chapter inheritance

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Merge pull request #4 from xiehuc/master

finish chapter inheritance