Remove Python 2 code

[hugovk]

Some extra cleanup for #390.

• Remove some if sys.version_info < (2, 4): code
• Run https://github.com/asottile/pyupgrade with --py3-plus
• Remove unnecessary __getslice__, deprecated since Python 2.0 and removed in 3

[codecov]

Codecov Report

Merging #410 into master will increase coverage by 7.36%.
The diff coverage is 95.08%.

@@            Coverage Diff             @@
##           master     #410      +/-   ##
==========================================
+ Coverage   80.81%   88.17%   +7.36%     
==========================================
  Files          22       24       +2     
  Lines        1647     2385     +738     
==========================================
+ Hits         1331     2103     +772     
+ Misses        316      282      -34

Impacted Files	Coverage Δ
src/tablib/packages/dbfpy/dbfnew.py	97.61% <ø> (ø)	⬆️
src/tablib/packages/dbfpy/utils.py	90% <ø> (+41.61%)	⬆️
src/tablib/formats/_rst.py	93.8% <100%> (ø)	⬆️
src/tablib/formats/_html.py	100% <100%> (ø)	⬆️
src/tablib/formats/_xlsx.py	83.52% <100%> (ø)	⬆️
tests/test_tablib_dbfpy_packages_utils.py	100% <100%> (ø)
src/tablib/formats/_jira.py	100% <100%> (ø)	⬆️
src/tablib/core.py	81.44% <20%> (+3.61%)	⬆️
src/tablib/packages/dbfpy/fields.py	67.81% <50%> (+0.2%)	⬆️
tests/test_tablib.py	98.35% <97.14%> (ø)
... and 4 more

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[claudep]

I don't care so much, but replacing % placeholder syntax by format() brings nothing AFAIK in our case (contrary to the Python 3.6+ f-string syntax which might help for readability). But if you like this, feel free to go ahead with this patch.

[hugovk]

format() is the more modern way in Python, and it's easier to use the pyupgrade defaults. We can run it again when dropping Python 3.5 and it'll do some f-string conversion too.

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Also added some more tests, and include tests in coverage: https://nedbatchelder.com/blog/201908/dont_omit_tests_from_coverage.html

[claudep]

Thanks, nice progress for the coverage!