Update xlsx import to read cell values instead of cell formulas

[dragonworks]

When importing an Excel xlsx file it reads the cell formulas instead of the cell values.

For example, if a cell has the formulas '=SUM(A1:A3)' it will read that string instead of the calculated number in Excel.

Passing data_only=True to openpyxl's load_workbook will load the cell values instead of formulas.

I think when importing an Excel spreadsheet it's the values that would be wanted and not the formulas?

[codecov[bot]]

Codecov Report

Merging #461 into master will increase coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #461      +/-   ##
==========================================
+ Coverage   90.42%   90.44%   +0.02%     
==========================================
  Files          28       28              
  Lines        2589     2606      +17     
==========================================
+ Hits         2341     2357      +16     
- Misses        248      249       +1     

Impacted Files	Coverage Δ
src/tablib/formats/_xlsx.py	96.73% <100.00%> (ø)
tests/test_tablib.py	98.53% <100.00%> (+0.01%)	:arrow_up:
src/tablib/formats/_xls.py	84.78% <0.00%> (+0.07%)	:arrow_up:

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[claudep]

I think that makes sense. Could you please add:

• a Changelog entry in HISTORY.md
• a note in docs/formats.rst under the xlsx header
• a test reading an xlsx file containing a formula

[dragonworks]

Thanks, I've updated it with those changes. Hope that's okay, I'm kinda new to this.

[claudep]

That's great, thanks! I pushed minor edits to your patch, if you don't mind.

[dragonworks]

Thanks, I appreciate it!