Add flake8 to earthpy

[lwasser]

this addresses #519

[codecov-io]

Codecov Report

Merging #520 into master will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #520      +/-   ##
==========================================
- Coverage   94.44%   94.42%   -0.02%     
==========================================
  Files          25       25              
  Lines        1583     1579       -4     
==========================================
- Hits         1495     1491       -4     
  Misses         88       88

Impacted Files	Coverage Δ
earthpy/tests/conftest.py	100% <ø> (ø)	⬆️
earthpy/tests/test_plot.py	100% <ø> (ø)	⬆️
earthpy/mask.py	100% <ø> (ø)	⬆️
earthpy/tests/test_crop_all.py	100% <ø> (ø)	⬆️
earthpy/tests/test_hist.py	99.25% <ø> (ø)	⬆️
earthpy/tests/test_plot_bands.py	100% <ø> (ø)	⬆️
earthpy/tests/test_stack_raster_tifs.py	100% <ø> (ø)	⬆️
earthpy/io.py	53.93% <ø> (ø)	⬆️
earthpy/tests/test_stack.py	100% <ø> (ø)	⬆️
earthpy/tests/test_crop_image.py	100% <ø> (ø)	⬆️
... and 5 more

---

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 e4bc8c5...b9a2979. Read the comment docs.

[lwasser]

ok flake8 has been implemented and CI is happy!! merging this and can close the associated issue.