chore: Update `iso8601` to `0.6`

[codecov[bot]]

Codecov Report

Merging #418 (3741b86) into master (44b7e54) will decrease coverage by 0.28%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #418      +/-   ##
==========================================
- Coverage   78.87%   78.59%   -0.28%     
==========================================
  Files          54       54              
  Lines        4488     4481       -7     
==========================================
- Hits         3540     3522      -18     
- Misses        948      959      +11     

Impacted Files	Coverage Δ
jsonschema/src/keywords/type_.rs	76.70% <0.00%> (-3.98%)	:arrow_down:
jsonschema/src/keywords/multiple_of.rs	77.58% <0.00%> (-3.45%)	:arrow_down:
jsonschema/src/keywords/additional_items.rs	76.05% <0.00%> (-1.41%)	:arrow_down:
jsonschema/src/keywords/legacy/type_draft_4.rs	68.47% <0.00%> (-1.09%)	:arrow_down:
jsonschema/src/resolver.rs	78.99% <0.00%> (-0.85%)	:arrow_down:
jsonschema/src/compilation/options.rs	84.77% <0.00%> (-0.38%)	:arrow_down:
jsonschema/src/schemas.rs	89.62% <0.00%> (-0.10%)	:arrow_down:
jsonschema/src/keywords/helpers.rs	62.50% <0.00%> (+1.27%)	:arrow_up:

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