Add support for monochromatic images (PIL image.mode == '1')

[GerardoAllende]

Added support for monochromatic images (PIL image.mode == '1')

Checklist:

• The GitHub pipeline is OK (green),
  meaning that both pylint (static code analyzer) and black (code formatter) are happy with the changes of this PR.

• A unit test is covering the code added / modified by this PR

• This PR is ready to be merged

• 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

By submitting this pull request, I confirm that my contribution is made under the terms of the GNU LGPL 3.0 license.

[Lucas-C]

This looks great, thank you for your contribution @GerardoAllende!
If the GitHub Actions checks pass, I'll merge this PR as it its!

@allcontributors please add @GerardoAllende for code

[allcontributors]

@Lucas-C

I've put up a pull request to add @GerardoAllende! 🎉

[Lucas-C]

Oh, seems like you haven't run black on the code:

Run black --check .
would reformat fpdf/image_parsing.py

Oh no! 💥 💔 💥
1 file would be reformatted, 134 files would be left unchanged.

Please follow those steps to fix this:
https://pyfpdf.github.io/fpdf2/Development.html#code-auto-formatting

[GerardoAllende]

Please follow those steps to fix this:
https://pyfpdf.github.io/fpdf2/Development.html#code-auto-formatting

Done

[codecov]

Codecov Report

Merging #527 (75533cc) into master (a2ed1d5) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #527   +/-   ##
=======================================
  Coverage   94.09%   94.10%           
=======================================
  Files          22       22           
  Lines        6200     6206    +6     
  Branches     1263     1265    +2     
=======================================
+ Hits         5834     5840    +6     
  Misses        191      191           
  Partials      175      175           

Impacted Files	Coverage Δ
fpdf/image_parsing.py	88.69% <100.00%> (+0.62%)	⬆️

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[Lucas-C]

#530 has been solved

Could you please rebase your branch on master @GerardoAllende?
That should solve the failing GitHub Action pipeline 😊

[Lucas-C]

Merged!
Thank you for your contribution @GerardoAllende

[GerardoAllende]

Thank you for your guidance and support!