support windows

[mszhanyi]

fix #393

1. 01_sync.ipynb and 03_export2html.ipynb failed in windows
2. The reasons are
   2.1 Unix path is not applicable on Windows
   2.2 Notebook could not use parallel on Windows

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[jph00]

Many thanks for the PR! Just one suggestion to try out please.

[jph00]

`parallel` uses fastai's ProcessPoolExecutor, not the stdlib one, which behaves differently with n_workers=0. Try it :)

[jph00]

@jph00 <https://github.com/jph00> , Thank your reply. it was stuck when if n_workers=0. I find it's caused by https://github.com/fastai/fastcore/blob/8ddf4dde118dea53daf849d7a5f2dc425ca49390/fastcore/parallel.py#L84-L85. The Manager().Lock() spawns a new process. Could we don't set self.lock = Manager().Lock() <https://www.fastmail.com/mail/url?u=35e541ae> when it is not_parallel?

Ah great find - yes feel free to submit a PR to add a condition there. (And please at-mention me on any PRs you do)

[mszhanyi]

git diff throws exception without https://github.com/mszhanyi/nbdev/blob/93ceef10b29e2d897f2e14d8ed1c81fbcab0bc53/nbdev/clean.py#L85

[jph00]

Super - thank you :)