Eliminate support for python 3.6

[rileyshahar]

We're going to have to write our own docker setup to run the autograder in, but it'll be worth it; this is also better because it means the tests aren't being run in an environment different from the python we encourage students to use.

This will resolve #25.

[codecov[bot]]

Codecov Report

Merging #28 (d35d627) into main (d48baab) will decrease coverage by 0.60%. The diff coverage is 93.54%.

@@             Coverage Diff             @@
##              main      #28      +/-   ##
===========================================
- Coverage   100.00%   99.39%   -0.61%     
===========================================
  Files            8        9       +1     
  Lines          312      332      +20     
  Branches        40       47       +7     
===========================================
+ Hits           312      330      +18     
- Misses           0        2       +2     

Impacted Files	Coverage Δ
src/aga/gradescope/resources/setup.py	0.00% <0.00%> (ø)
src/aga/loader.py	100.00% <ø> (ø)
src/aga/__init__.py	100.00% <100.00%> (ø)
src/aga/gradescope/into_zip.py	100.00% <100.00%> (ø)

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[rileyshahar]

The current issue with this is with testing: currently, we load the library from pip inside the testing environment. This is problematic for incompatible versions of the library, and in particular is preventing testing this on the actual gradescope server.