Add parameter for pre-release dependencies

Author: gclendenning

azure-pipelines.yml

@@ -17,6 +17,12 @@ pr:
     - doc/*
     - README.rst
 
+parameters:
+  - name: includeReleaseCandidates
+    displayName: "Allow pre-release dependencies"
+    type: boolean
+    default: false
+
 variables:
   triggeredByPullRequest: $[eq(variables['Build.Reason'], 'PullRequest')]
 
@@ -66,8 +72,17 @@ stages:
 
         - script: |
             python -m pip install --upgrade pip
+          displayName: 'Upgrade pip'
+
+        - script: |
             pip install -r requirements.txt
           displayName: 'Install dependencies'
+          condition: ${{ eq(parameters.includeReleaseCandidates, false) }}
+
+        - script: |
+            pip install --pre -r requirements.txt
+          displayName: 'Install dependencies (allow pre-releases)'
+          condition: ${{ eq(parameters.includeReleaseCandidates, true) }}
 
         - script: |
             pip install -e .