azure.storage.blob 12.x compatability

[TomAugspurger]

https://pypi.org/project/azure-storage-blob/ 12.x was recently released. Some things broke.

In [1]: import adlfs
---------------------------------------------------------------------------
ImportError                               Traceback (most recent call last)
<ipython-input-1-fd1e7da2cebf> in <module>
----> 1 import adlfs

~/sandbox/dask-adlfs/adlfs/__init__.py in <module>
----> 1 from .core import AzureDatalakeFileSystem
      2 from .core import AzureBlobFileSystem, AzureBlobFile
      3
      4 __all__ = ["AzureBlobFileSystem", "AzureBlobFile", "AzureDatalakeFileSystem"]

~/sandbox/dask-adlfs/adlfs/core.py in <module>
      7 from azure.datalake.store import lib, AzureDLFileSystem
      8 from azure.datalake.store.core import AzureDLPath, AzureDLFile
----> 9 from azure.storage.blob import BlockBlobService
     10 from fsspec import AbstractFileSystem
     11 from fsspec.spec import AbstractBufferedFile

ImportError: cannot import name 'BlockBlobService' from 'azure.storage.blob' (/Users/taugspurger/.virtualenvs/filesystems/lib/python3.7/site-packages/azure/storage/blob/__init__.py)

[AlbertDeFusco]

I think I'll need to check this with my changes

[hayesgb]

For the time being, we've pinned the version of the azure-storage package to <3.0

[hayesgb]

Recent push to master uses azure storage blob. Need to pin in requirements.