Stop using @master for GH actions (#5503)

Author: alex

.github/workflows/ci.yml

@@ -20,7 +20,7 @@ jobs:
           - {VERSION: "3.9", TOXENV: "py39", EXTRA_CFLAGS: "-DUSE_OSRANDOM_RNG_FOR_TESTING"}
     name: "Python ${{ matrix.PYTHON.VERSION }} on macOS"
     steps:
-      - uses: actions/checkout@master
+      - uses: actions/checkout@v2
       - name: Setup python
         uses: actions/setup-python@v2
         with:
@@ -66,7 +66,7 @@ jobs:
           - {VERSION: "3.9", TOXENV: "py39", MSVC_VERSION: "2019", CL_FLAGS: "/D USE_OSRANDOM_RNG_FOR_TESTING"}
     name: "Python ${{ matrix.PYTHON.VERSION }} on ${{ matrix.WINDOWS.WINDOWS }}"
     steps:
-      - uses: actions/checkout@master
+      - uses: actions/checkout@v2
       - name: Setup python
         uses: actions/setup-python@v2
         with:

.github/workflows/wheel-builder.yml

@@ -67,7 +67,7 @@ jobs:
             BIN_PATH: '/Library/Frameworks/Python.framework/Versions/3.8/bin/python3'
     name: "${{ matrix.PYTHON.VERSION }} ABI ${{ matrix.PYTHON.ABI_VERSION }} macOS"
     steps:
-      - uses: actions/checkout@master
+      - uses: actions/checkout@v2
       - run: |
           curl "$PYTHON_DOWNLOAD_URL" -o python.pkg
           sudo installer -pkg python.pkg -target /
@@ -122,7 +122,7 @@ jobs:
           - {VERSION: "3.8", MSVC_VERSION: "2019", "USE_ABI3": "true", "ABI_VERSION": "cp36"}
     name: "${{ matrix.PYTHON.VERSION }} ${{ matrix.WINDOWS.WINDOWS }} ${{ matrix.PYTHON.ABI_VERSION }}"
     steps:
-      - uses: actions/checkout@master
+      - uses: actions/checkout@v2
       - name: Setup python
         uses: actions/setup-python@v2
         with: