style: Added encoding in headers

[frgfm]

This PR aims at:

• adding utf8 encoding when it was missing in header to avoid issues
• switched to Python 3 class definitions style
• switched from absolute to relative import when it was not needed (absolute could use existing package installation even if they do not share the same codebase)

[codecov]

Codecov Report

Merging #55 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #55   +/-   ##
=======================================
  Coverage   83.80%   83.80%           
=======================================
  Files          18       18           
  Lines         704      704           
=======================================
  Hits          590      590           
  Misses        114      114           

Impacted Files	Coverage Δ
pyronear/datasets/openfire.py	89.47% <ø> (ø)
pyronear/datasets/utils.py	91.66% <ø> (ø)
pyronear/datasets/wildfire/fire_labeler.py	100.00% <ø> (ø)
pyronear/datasets/wildfire/split_strategy.py	96.77% <ø> (ø)
pyronear/models/densenet.py	58.53% <ø> (ø)
pyronear/models/mobilenet.py	63.63% <ø> (ø)
pyronear/models/resnet.py	82.60% <ø> (ø)
pyronear/models/utils.py	98.18% <ø> (ø)
pyronear/nn/functional.py	100.00% <ø> (ø)
pyronear/nn/modules/pooling.py	90.90% <ø> (ø)
... and 1 more

[x0s]

Thanks for the PR. Quickly:

• Class definition are already new-style
• The files concerned in this PR are not intended to be used as scripts
• Okay for relative imports

[frgfm]

Thanks for the PR. Quickly:

• Class definition are already new-style
• The files concerned in this PR are not intended to be used as scripts
• Okay for relative imports

Thanks for the review, just made the suggested modifications!