CI: Run tests in a single CI job (Ubuntu + Python 3.9) for draft PRs (#906)

This PR improves the "Tests" workflow to save CI resources:

- For draft PRs: only run tests on Ubuntu + Python 3.9 (~10 minutes)
- When draft PRs are ready for review, run all CI jobs (9 jobs, 10-30 minutes)
- If converting PRs back to draft mode, pushes still only trigger one job (~10 minutes)
- After merged into master, all 9 jobs will run.

* Update MAINTENANCE.md

Co-authored-by: Wei Ji <23487320+weiji14@users.noreply.github.com>

Author: seisman

.github/workflows/ci_tests.yaml

@@ -7,6 +7,7 @@ on:
   push:
     branches: [ master ]
   pull_request:
+    types: [opened, reopened, synchronize, ready_for_review]
     paths-ignore:
       - 'doc/**'
       - '*.md'
@@ -29,6 +30,22 @@ jobs:
       matrix:
         python-version: [3.7, 3.8, 3.9]
         os: [ubuntu-latest, macOS-latest, windows-latest]
+        # Is it a draft Pull Request (true or false)?
+        isDraft:
+          - ${{ github.event.pull_request.draft }}
+        # Only run one job (Ubuntu + Python 3.9) for draft PRs
+        exclude:
+          - os: macOS-latest
+            isDraft: true
+          - os: windows-latest
+            isDraft: true
+          - os: ubuntu-latest
+            python-version: 3.7
+            isDraft: true
+          - os: ubuntu-latest
+            python-version: 3.8
+            isDraft: true
+
     # environmental variables used in coverage
     env:
       OS: ${{ matrix.os }}

MAINTENANCE.md

@@ -58,6 +58,8 @@ It is also scheduled to run daily on the *master* branch.
 
 This is ran on every commit to the *master* and Pull Request branches.
 It is also scheduled to run daily on the *master* branch.
+In draft Pull Requests, only one job (Ubuntu + Python latest)
+is triggered to save on Continuous Integration resources.
 
 On the *master* branch, the workflow also handles the documentation deployment: