Skip Project formatting on Julia <1.7 if there are weakdeps

[gdalle]

Brutal fix for #208. If the Julia version is below 1.7 and the Project.toml contains weakdeps or extensions, its formatting test will now return true by default.
This only affects people who write package extensions, and my assumption is that those people test on 1.9 and above anyway.

See also JuliaLang/Pkg.jl#3481

Ping @adrhill

closes #209, closes #210

[codecov]

Codecov Report

Merging #206 (d2d62f8) into master (19a5588) will increase coverage by 0.06%.
Report is 1 commits behind head on master.
The diff coverage is 100.00%.

❗ Current head d2d62f8 differs from pull request most recent head 801677d. Consider uploading reports for the commit 801677d to get more accurate results

@@            Coverage Diff             @@
##           master     #206      +/-   ##
==========================================
+ Coverage   76.80%   76.86%   +0.06%     
==========================================
  Files          11       11              
  Lines         763      765       +2     
==========================================
+ Hits          586      588       +2     
  Misses        177      177              

Flag	Coverage Δ
unittests	76.86% <100.00%> (+0.06%)	⬆️

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Files	Coverage Δ
src/project_toml_formatting.jl	86.48% <100.00%> (+0.77%)	⬆️

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