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Bump test version of azure-storage-blob

Bump test version of azure-storage-blob

Author: dstansby

requirements_dev_optional.txt

@@ -5,9 +5,7 @@ lmdb==1.5.1; sys_platform != 'win32'
 ipytree==0.2.2
 ipywidgets==8.1.3
 # optional library requirements for services
-# don't let pyup change pinning for azure-storage-blob, need to pin to older
-# version to get compatibility with azure storage emulator on appveyor (FIXME)
-azure-storage-blob==12.16.0 # pyup: ignore
+azure-storage-blob==12.21.0
 redis==5.0.8
 types-redis
 types-setuptools