Add scripts to modernize to Python 3

[Eric-Arellano]

These scripts were developed to modernize https://github.com/pantsbuild/pants as it drops Python 2 and can start to use Python 3-only features.

• remove_builtins.py unravels part of the future library by removing all from builtins imports, which no-op in Py3. Its main innovations are the CLI interface and removing BUILD entries if possible.
• update_headers.py removes from __future__ imports and # coding=utf-8 lines, which both no-op on Python 3.
• update_decode_encode.py uses the default utf-8 encoding to simplify string calls to encode() and decode().
• modernize_classes.py simplifies calls to super() and removes the unnecessary object base class, as all classes are new style in Python 3.

[Eric-Arellano]

cc @OniOni, @mateor, @GoingTharn

I tried these on a couple sample folders in Pants repo, but haven't actually fully used them yet because Pants is still a few days away from dropping Py2. You might want to wait to merge this until I get those upstream Pants PRs up, in case I have to make any modifications to these scripts.

[mateor]

Thanks Eric! As long as they don't break CI, you can choose to land these any way you prefer, incrementally or otherwise.

Is this something you think should go on PyPi? We could publish whls at some point, if you think it could work

[Eric-Arellano]

you can choose to land these any way you prefer, incrementally or otherwise.

Great. I know update_headers.py is good to go, but haven't used remove_builtins.py at scale yet so lets wait for that one. I'll give the go-ahead when ready.

Is this something you think should go on PyPi? We could publish whls at some point, if you think it could work

No I don't think so. These scripts are far too specific to Pants and not general enough to be wheels. They are also meant to be one-time scripts that you never run again, so the expectation is that people would copy this script into their local directory, run it, then delete it, rather than to download a wheel.

[OniOni]

No I don't think so. These scripts are far too specific to Pants and not general enough to be wheels. They are also meant to be one-time scripts that you never run again, so the expectation is that people would copy this script into their local directory, run it, then delete it, rather than to download a wheel.

Agreed, it would also be a non trivial refactor for something that works just as well as a script.

[Eric-Arellano]

This is now ready to merge. Used both scripts on upstream Pants this week and made substantial changes as a result.

[Eric-Arellano]

Actually we can hold off on merging for now. I'm going to write a couple more scripts like to no longer use class Foo(object). Will put them all in the same PR.

[Eric-Arellano]

This is now ready to land.

[Eric-Arellano]

bump @OniOni @mateor ready to land

[OniOni]

Oh sweet! I'll take a look!

[OniOni]

Thanks @Eric-Arellano here it is: 332e7a22526263f154dd0ac3c25389251570120d

[mateor]

Ah - @Eric-Arellano++

I see this on master, always great to see your work!