Add __version__ variable

[nils-braun]

Fixes #74 .
As dask-sql uses version tags via git tags, we can just use the importlib.metadata version here.

[codecov-io]

Codecov Report

Merging #79 (dc333a5) into main (3507ad9) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #79   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           32        33    +1     
  Lines         1356      1362    +6     
  Branches       184       184           
=========================================
+ Hits          1356      1362    +6     

Impacted Files	Coverage Δ
dask_sql/_version.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3507ad9...dc333a5. Read the comment docs.