python: fix to_parity default option handling

* python/spot/__init__.py (to_options): Do not have options
explicitely default to to_parity_options(), because that would be
instantiated only once.
14 jobs for next in 153 minutes and 50 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #89753
alpine-gcc

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passed #89754
arch-clang

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arch-gcc-glibcxxdebug

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debian-gcc-snapshot

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passed #89750
debian-stable-gcc

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passed #89751
debian-unstable-gcc-coverage

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90.5%
passed #89758
debpkg-stable

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passed #89759
debpkg-unstable

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passed #89756
mingw-shared

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passed #89757
mingw-static

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passed #89761
armv7
raspbian

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rpm-pkg

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  Publish
passed #89762
dput
publish-rpm

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passed #89763
dput
publish-unstable

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