Change License to Apache 2.0

[fubuloubu]

This PR changes our license from MIT to Apache 2.0, applicable for all future versions of Vyper.

Being a move from a less permissive license to a more permissive license, there is no requirement to obtain approval from previous contributors as your copyright extends even after this change has been made.

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More about licenses:

• MIT
• Apache 2.0

[codecov-io]

Codecov Report

Merging #1772 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1772   +/-   ##
=======================================
  Coverage   86.32%   86.32%           
=======================================
  Files          51       51           
  Lines        5982     5982           
  Branches     1554     1554           
=======================================
  Hits         5164     5164           
  Misses        530      530           
  Partials      288      288

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