feat: add FileContents to read/watch files

[manzt]

Closes #61

This PR adds the anywidget._file_contents.FileContents object. FileContents watches for file changes in a provided path in a background thread and continuously emits changed signals any time the file changes, until the file is deleted (emits a deleted event). Connecting FileContents with MimeReprBuilder or AnyWidget allows for #60 to be leveraged during development. Currently this must be done manually, but we should aim to make the creation of FileContents hidden.

With traitlets

import anywidget
import traitlets
from anywidget._file_contents import FileContents

contents = FileContents("index.js")

class Counter(anywidget.AnyWidget):
    _esm = str(contents)
    value = traitlets.Int(0).tag(sync=True)

counter = Counter()
contents.changed.connect(lambda change: setattr(counter, "_esm", change)
counter

With MimeReprBuilder

import anywidget
from anywidget._file_contents import FileContents
from anywidget._descriptor_ import MimeReprBuilder

contents = FileContents("index.js")

@evented
@dataclasses
class Counter:
    _repr_mimebundle_ = MimeReprBuilder(_esm=str(contents)
    value = traitlets.Int(0).tag(sync=True)

counter = Counter()
contents.changed.connect(lambda change: counter._repr_mimebundle_._send_hmr_update(esm=change))
counter

[netlify]

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[codecov]

Codecov Report

Merging #62 (10d88d3) into main (5b1c466) will decrease coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #62      +/-   ##
==========================================
- Coverage   98.34%   98.28%   -0.07%     
==========================================
  Files           6        7       +1     
  Lines         303      349      +46     
==========================================
+ Hits          298      343      +45     
- Misses          5        6       +1     

Impacted Files	Coverage Δ
anywidget/_file_contents.py	100.00% <100.00%> (ø)
anywidget/_descriptor.py	98.93% <0.00%> (-0.54%)	⬇️

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[manzt]

Lol, not quite sure why the coverage is still messaged up. Anyways, I think this is probably ready for a review now. Like I said, still thinking about the right API to hide the FileContents for anywidget but I think that should probably be addressed in a subsequent PR.

[tlambert03]

(still looking)

[tlambert03]

this is great. thanks for writing all the tests.

merge when ready (s'ok if you either want to revert my suggestion, or use the sleep approach)

[manzt]

Thanks for the review! I tried the sleep but it failed in CI, so I'm reverting :) Learned a lot about python unit testing with this PR lol. Love it. thanks!