Fix the PickleError with RawDeltaTable

[j-bennet]

Fixes an error when reading a Dask dataframe with dask-deltalake and distributed.Client.

<dask.highlevelgraph.HighLevelGraph object at 0x13fac3fd0>
 0. read-delta-table-f039f9f4fa4ebf9b2dd57eb38dcfa70e
>.
Traceback (most recent call last):
  File "/Users/jbennet/mambaforge/envs/dask-deltatable/lib/python3.9/site-packages/distributed/protocol/pickle.py", line 63, in dumps
    result = pickle.dumps(x, **dump_kwargs)
TypeError: cannot pickle 'builtins.RawDeltaTable' object

This PR looks like a lot of changes, but that's only because I moved the data fixtures to conftest.py so they can be shared between modules. The real changes are all in core.py.

cc @fjetter

Closes https://github.com/dask-contrib/dask-deltatable/issues/56.

[codecov-commenter]

Codecov Report

Merging #57 (a8f1e56) into main (dbeb8cc) will increase coverage by 0.15%. The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main      #57      +/-   ##
==========================================
+ Coverage   74.61%   74.76%   +0.15%     
==========================================
  Files           6        6              
  Lines         323      325       +2     
==========================================
+ Hits          241      243       +2     
  Misses         82       82              

Impacted Files	Coverage Δ
dask_deltatable/core.py	91.76% <100.00%> (+0.19%)	:arrow_up:

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[j-bennet]

Thanks @fjetter @jrbourbeau , I'll rework this. Hopefully that will help with performance, too.

[j-bennet]

@fjetter It's green.