Support Python 3(@rmatev)

Commits cherry-picked from !138 (merged)

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• [2020-09-22 00:12] Validation started with lhcb-run2-patches-py3#49
• [2020-09-23 00:13] Validation started with lhcb-run2-patches-py3#50
• [2020-09-24 00:20] Validation started with lhcb-run2-patches-py3#51
• [2020-09-24 11:28] Validation started with lhcb-run2-patches-py3#52
• [2020-09-25 00:11] Validation started with lhcb-run2-patches-dev4#461
• [2020-09-25 00:12] Validation started with lhcb-run2-patches-py3#53
• [2020-09-25 00:22] Validation started with lhcb-run2-patches#825

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• 72a21414 - Update Configuration.py to fix one more print statement

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• 8838e0d4 - Tweaks of futurize --stage1, not applied in cherry-picks from master

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• 7c2dd1fb - Tweaks of futurize --stage1, not applied in cherry-picks from master

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• b1a32041 - Tweaks of futurize --stage1, not applied in cherry-picks from master

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This MR is ready to go. @rmatev could you please merge it, as I don't have the Karma...

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merged

mentioned in commit f17addb2

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