Update license definitions to include MIT and Dask's BSD3

[djhoese]

Possibly closes #118. So in hindsight I should have just made this library BSD3 and it probably would have been simpler or at least less blurry. CC @qulogic

This PR updates how the package metadata defines the license. I wanted this library to be MIT, not realizing at the time that dask was not MIT but BSD3. As far as I understand license headers are not necessary in each source file so I've removed the existing ones to avoid confusion about what license a particular source file was under. The copyright comment is also apparently optional under MIT and BSD3, but I've kept the copyright comments to make it more clear which files are from dask (Anaconda copyright) or created just for this library (Donfig copyright or no explicit copyright).

So in summary I think this library is defined as being MIT and BSD3 licensed. Code was and is taken from Dask and then put into Donfig. Since I'm keeping the original BSD3 license I'm not sure what that says about the license of the modified versions of that code.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 98.28%. Comparing base (7b90e4d) to head (1752257). :warning: Report is 46 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #127      +/-   ##
==========================================
- Coverage   98.28%   98.28%   -0.01%     
==========================================
  Files           6        6              
  Lines         759      758       -1     
==========================================
- Hits          746      745       -1     
  Misses         13       13              

Flag	Coverage Δ
unittests	98.28% <100.00%> (-0.01%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

:rocket: New features to boost your workflow:

• :snowflake: Test Analytics: Detect flaky tests, report on failures, and find test suite problems.