Clean up some IDE warnings

[hugovk]

Some cleanup to remove some warnings shown in PyCharm.

See commit messages for details.

[codecov]

Codecov Report

Merging #559 (3f74f1c) into master (f3ef2e9) will increase coverage by 0.22%.
Report is 2 commits behind head on master.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #559      +/-   ##
==========================================
+ Coverage   91.37%   91.59%   +0.22%     
==========================================
  Files          28       28              
  Lines        2713     2785      +72     
==========================================
+ Hits         2479     2551      +72     
  Misses        234      234              

Files Changed	Coverage Δ
src/tablib/formats/_xlsx.py	100.00% <ø> (ø)
src/tablib/packages/dbfpy/dbfnew.py	97.67% <ø> (ø)
src/tablib/__init__.py	60.00% <100.00%> (ø)
src/tablib/core.py	85.01% <100.00%> (ø)
src/tablib/formats/__init__.py	95.45% <100.00%> (ø)
src/tablib/formats/_dbf.py	100.00% <100.00%> (ø)
src/tablib/formats/_df.py	75.00% <100.00%> (ø)
src/tablib/formats/_html.py	100.00% <100.00%> (ø)
src/tablib/formats/_latex.py	100.00% <100.00%> (ø)
src/tablib/formats/_ods.py	98.18% <100.00%> (ø)
... and 3 more

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[claudep]

Looks good!