Update `pronto` and drop Python 2 compatibility layer

[althonos]

This PR updates pronto, but since v2 it doesn't support Python 2 anymore, so I also removed the compatibility code for Python 2.

Changes

• Updated local PSI-MS file with latest release.
• Replaced pkg-resources with importlib.resources for loading the embedded files.
• Updated pronto, and added cache for subclass queries since the new versions of pronto don't cache it anymore.
• Replaced cached_property with functools.cached_property if available in Python stdlib.
• Removed six dependency and all Python 2/3 compatibility code.

[codecov-commenter]

Codecov Report

Merging #48 (1b138b0) into master (acf4220) will increase coverage by 0.68%. The diff coverage is 94.36%.

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
+ Coverage   75.44%   76.12%   +0.68%     
==========================================
  Files          12       12              
  Lines         843      842       -1     
==========================================
+ Hits          636      641       +5     
+ Misses        207      201       -6     

Flag	Coverage Δ
3.6	74.90% <94.36%> (+0.24%)	:arrow_up:
3.7	74.66% <91.54%> (ø)
3.8	75.17% <88.73%> (-0.27%)	:arrow_down:
3.9	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mzml2isa/__init__.py	80.00% <ø> (-3.34%)	:arrow_down:
mzml2isa/parsing.py	32.50% <ø> (-0.84%)	:arrow_down:
mzml2isa/usermeta.py	18.51% <0.00%> (-1.97%)	:arrow_down:
mzml2isa/utils.py	41.93% <ø> (-10.57%)	:arrow_down:
tests/test_mtbls.py	96.29% <ø> (-0.05%)	:arrow_down:
mzml2isa/mzml.py	93.13% <94.87%> (+0.11%)	:arrow_up:
mzml2isa/_impl.py	75.00% <100.00%> (+35.00%)	:arrow_up:
mzml2isa/imzml.py	76.19% <100.00%> (+3.96%)	:arrow_up:
mzml2isa/isa.py	93.63% <100.00%> (+0.11%)	:arrow_up:
tests/_utils.py	100.00% <100.00%> (+15.38%)	:arrow_up:
... and 2 more

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[Tomnl]

Thanks @althonos !!