Deprecating txt= arg in favor of text= - solves #853

[Lucas-C]

This MR aims to harmonize argument naming.
Before this MR:

• FPDF.cell(), FPDF.multi_cell(), FPDF.text() & FPDF.write() accepted a txt= argument
• FPDF.write_html() & table.Row.cell() accepted a text= argument

After merging this PR:

• all public fpdf2 methods that accepted txt= will now also accept text=
• text= will be the documented & recommended way to pass this argument
• passing txt= will still work but generate a DeprecationWarning

The decorator idea came from a suggestion by @gmischler there: #897 (comment)

⚠️ This PR should probably be merged just before releasing, to avoid having our online documentation being updated with text mentioned in method docstrings, whereas they do no support this argument in v2.7.5

[gmischler]

Looking good!

You're probably correct about merging just before release. Although given the sweeping nature of the change, interference with other PRs is likely but hard to avoid.

[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (e918119) 93.63% compared to head (7d5f6a6) 93.61%.

❗ Current head 7d5f6a6 differs from pull request most recent head e0123f1. Consider uploading reports for the commit e0123f1 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #903      +/-   ##
==========================================
- Coverage   93.63%   93.61%   -0.02%     
==========================================
  Files          28       28              
  Lines        8211     8227      +16     
  Branches     1501     1503       +2     
==========================================
+ Hits         7688     7702      +14     
- Misses        324      325       +1     
- Partials      199      200       +1     

Files	Coverage Δ
fpdf/html.py	95.89% <100.00%> (ø)
fpdf/table.py	94.24% <ø> (ø)
fpdf/template.py	90.39% <100.00%> (ø)
fpdf/fpdf.py	92.74% <97.87%> (+0.01%)	⬆️
fpdf/deprecation.py	93.02% <83.33%> (-3.76%)	⬇️

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