Group tests with similar duration together

Style and documentation tests take just a few minutes
to run. Since in Github actions one can't restart a single
job but needs to restart an entire workflow, here we group
tests with similar duration together.

.github/workflows/style_and_docs.yaml

@@ -0,0 +1,70 @@
+name: style and docs
+
+on:
+  push:
+    branches:
+      - develop
+      - releases/**
+  pull_request:
+    branches:
+      - develop
+      - releases/**
+jobs:
+  validate:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Setup Python
+      uses: actions/setup-python@v1
+      with:
+        python-version: 3.7
+    - name: Install Python Packages
+      run: |
+        pip install --upgrade pip
+        pip install --upgrade vermin
+    - name: Minimum Version (Spack's Core)
+      run: vermin --backport argparse -t=2.6- -t=3.5- -v lib/spack/spack/ lib/spack/llnl/ bin/
+    - name: Minimum Version (Repositories)
+      run: vermin --backport argparse -t=2.6- -t=3.5- -v var/spack/repos
+  flake8:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+      with:
+        fetch-depth: 0
+    - uses: actions/setup-python@v2
+      with:
+        python-version: 3.8
+    - name: Install Python packages
+      run: |
+        pip install --upgrade pip six setuptools flake8
+    - name: Setup git configuration
+      run: |
+        # Need this for the git tests to succeed.
+        git --version
+        git config --global user.email "spack@example.com"
+        git config --global user.name "Test User"
+        git fetch -u origin develop:develop
+    - name: Run flake8 tests
+      run: |
+          share/spack/qa/run-flake8-tests
+  documentation:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: actions/setup-python@v2
+      with:
+        python-version: 3.8
+    - name: Install System packages
+      run: |
+        sudo apt-get -y update
+        sudo apt-get install -y coreutils ninja-build graphviz
+    - name: Install Python packages
+      run: |
+        pip install --upgrade pip six setuptools
+        pip install --upgrade -r lib/spack/docs/requirements.txt
+    - name: Build documentation
+      run: |
+          share/spack/qa/run-doc-tests