Update scikit-image requirement (#551)

Updates the requirements on [scikit-image](https://github.com/scikit-image/scikit-image) to permit the latest version.
- [Release notes](https://github.com/scikit-image/scikit-image/releases)
- [Changelog](https://github.com/scikit-image/scikit-image/blob/main/RELEASE.txt)
- [Commits](scikit-image/scikit-image@v0.14.2...v0.19.0)

---
updated-dependencies:
- dependency-name: scikit-image
  dependency-type: direct:production
...

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setup.py

@@ -36,7 +36,7 @@ def readme():
           'pandas>=0.23.3, <2.0.0',
           'scikit-learn>=0.20.2, <1.1.0',
           'spacy[lookups]>=2.0.0, <4.0.0',
-          'scikit-image>=0.14.2, !=0.17.1, <0.19',  # https://github.com/SeldonIO/alibi/issues/215
+          'scikit-image>=0.14.2, !=0.17.1, <0.20',  # https://github.com/SeldonIO/alibi/issues/215
           'requests>=2.21.0, <3.0.0',
           'Pillow>=5.4.1, <10.0',
           'tensorflow>=2.0.0, !=2.6.0, !=2.6.1, <2.8.0',  # https://github.com/SeldonIO/alibi-detect/issues/375