Deprecating font caching mechanism + support for .pkl font files definitions

[Lucas-C]

cf. #345

Checklist:

• The GitHub pipeline is OK (green)

• Unit tests are covering the code added / modified by this PR

• This PR is ready to be merged

• [NA] In case of a new feature, docstrings have been added, with also some documentation in the docs/ folder

• A mention of the change is present in CHANGELOG.md

[codecov]

Codecov Report

Merging #347 (95ee573) into master (6dedf61) will increase coverage by 0.20%.
The diff coverage is 77.27%.

@@            Coverage Diff             @@
##           master     #347      +/-   ##
==========================================
+ Coverage   90.38%   90.58%   +0.20%     
==========================================
  Files          20       20              
  Lines        5926     5885      -41     
  Branches     1192     1182      -10     
==========================================
- Hits         5356     5331      -25     
+ Misses        346      331      -15     
+ Partials      224      223       -1     

Impacted Files	Coverage Δ
fpdf/template.py	93.23% <ø> (ø)
fpdf/fpdf.py	85.87% <77.27%> (+0.53%)	⬆️

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[Spenhouet]

The change looks good to me. Only one small remark on a test.

[Lucas-C]

Thansk for the review @Spenhouet!