Fix page size auto-detect failing when using write with default HPGL device

[abey79]

Description

Fixes #319

Checklist

• feature/fix implemented
• code formatting ok (black and isort)
• mypy vpype vpype_cli tests returns no error
• tests added/updated and pytest succeeds
• documentation added/updated
  • command docstring and option/argument help
  • README.md updated (Feature Overview)
  • CHANGELOG.md updated
  • RTD doc updated and building with no error (make clean && make html in docs/)

[codecov]

Codecov Report

Merging #328 (2e7cb41) into master (5f3fd97) will increase coverage by 0.23%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #328      +/-   ##
==========================================
+ Coverage   92.08%   92.31%   +0.23%     
==========================================
  Files          50       50              
  Lines        3954     3970      +16     
  Branches      521      521              
==========================================
+ Hits         3641     3665      +24     
+ Misses        209      203       -6     
+ Partials      104      102       -2     

Impacted Files	Coverage Δ
tests/test_hpgl.py	100.00% <100.00%> (ø)
vpype/config.py	97.87% <100.00%> (+6.56%)	⬆️
vpype/io.py	78.15% <100.00%> (+0.52%)	⬆️

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[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication