Remove object subclasses from Python classes.

[ChrisCummins]

The minimum supported Python version for CompilerGym is
3.6. Subclassing from object is only required for Python versions < 3.

[codecov-commenter]

Codecov Report

Merging #250 (731bf43) into development (e9d0937) will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##           development     #250   +/-   ##
============================================
  Coverage        81.02%   81.02%           
============================================
  Files               76       76           
  Lines             4337     4337           
============================================
  Hits              3514     3514           
  Misses             823      823           

Impacted Files	Coverage Δ
compiler_gym/datasets/benchmark.py	88.17% <100.00%> (ø)
compiler_gym/datasets/dataset.py	83.62% <100.00%> (ø)
compiler_gym/datasets/datasets.py	93.15% <100.00%> (ø)
compiler_gym/envs/llvm/llvm_benchmark.py	87.40% <100.00%> (ø)
compiler_gym/service/connection.py	74.15% <100.00%> (ø)
compiler_gym/third_party/inst2vec/__init__.py	93.33% <100.00%> (ø)
compiler_gym/util/shell_format.py	100.00% <100.00%> (ø)
compiler_gym/util/timer.py	100.00% <100.00%> (ø)
compiler_gym/views/observation.py	92.85% <100.00%> (ø)
compiler_gym/views/observation_space_spec.py	83.52% <100.00%> (ø)
... and 1 more

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