Add thead and tbody tags

[hugovk]

Based on kxxoling/PTable#10.

Thanks to @Sewci0 for the original PR:

Lack of and tags in _get_formatted_html_string and _get_simple_html_string made it quite hard to style using CSS.

[codecov]

Codecov Report

Merging #81 (c96f072) into master (f9913bb) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #81      +/-   ##
==========================================
+ Coverage   87.31%   87.38%   +0.06%     
==========================================
  Files           3        3              
  Lines        1498     1506       +8     
==========================================
+ Hits         1308     1316       +8     
  Misses        190      190              

Flag	Coverage Δ
GHA_Ubuntu	87.38% <100.00%> (+0.06%)	⬆️
GHA_Windows	87.38% <100.00%> (+0.06%)	⬆️
GHA_macOS	87.38% <100.00%> (+0.06%)	⬆️

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Impacted Files	Coverage Δ
src/prettytable/prettytable.py	82.39% <100.00%> (+0.13%)	⬆️
tests/test_prettytable.py	100.00% <100.00%> (ø)

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