ci/azure-build-and-test.yml: fix builds as Python 3.9 rolls out

Azure's conda has updated to Python 3.9, but astropy isn't yet available
for it, so we need to explicitly specify an older version to get it for
the docs and coverage builds.

ci/azure-build-and-test.yml

@@ -84,7 +84,7 @@ jobs:
       set -euo pipefail
       source activate-conda.sh
       set -x
-      \conda create -y -n build setuptools pip
+      \conda create -y -n build setuptools pip python=3.8
       conda activate build
       pip install $BASH_WORKSPACE/sdist/*.tar.gz
     displayName: Install from sdist
@@ -124,7 +124,7 @@ jobs:
       set -euo pipefail
       source activate-conda.sh
       set -x
-      \conda create -y -n build setuptools pip
+      \conda create -y -n build setuptools pip python=3.8
       conda activate build
       pip install $BASH_WORKSPACE/sdist/*.tar.gz
     displayName: Install from sdist