reload sys.path on job start

[gwenzek]

I propose the following to solve #12

I think that the less surprising behavior possible.
We restore sys.path to whatever it was when we created the job instance.

In practice the two paths only differs on the first entry, but I'm not sure we want to assert that.
The scheduler script generally has the folder of scheduler script as first entry while the submitted job has the current folder.

Ouput of test_submitit_respects_sys_path if you skip the sys.path reloading:

E         - /private/var/folders/db/tkc8c7p17xzgpf_kv6ybsdcdng_9j1/T/pytest-of-guw/pytest-81/test_submitit_respects_sys_pat0
E         + /Users/guw/Documents/github/submitit
E           /opt/homebrew/Cellar/python37/3.7.5_3/Frameworks/Python.framework/Versions/3.7/lib/python37.zip
E           /opt/homebrew/Cellar/python37/3.7.5_3/Frameworks/Python.framework/Versions/3.7/lib/python3.7
E           /opt/homebrew/Cellar/python37/3.7.5_3/Frameworks/Python.framework/Versions/3.7/lib/python3.7/lib-dynload
E           /Users/guw/Documents/github/submitit/venv/lib/python3.7/site-packages
E           /Users/guw/Documents/github/submitit

[jrapin]

you should probably hide it, and use a tuple to make it unmutable.
I am not sure I like it. I am actually pretty sure I don't :D but if that fixes it, then why not (modulo the other comment...)

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