Use RC version for Python 3.10

Apparently, 3.10-dev isn't available anymore?

Author: sampsyo

.github/workflows/ci.yaml

@@ -12,7 +12,7 @@ jobs:
     strategy:
       matrix:
         platform: [ubuntu-latest, windows-latest]
-        python-version: [3.6, 3.7, 3.8, 3.9, 3.10-dev]
+        python-version: [3.6, 3.7, 3.8, 3.9, 3.10.0-rc.2]
 
     env:
       PY_COLORS: 1
@@ -45,7 +45,7 @@ jobs:
           sudo apt install ffmpeg  # For replaygain
 
       - name: Test older Python versions with tox
-        if: matrix.python-version != '3.9' && matrix.python-version != '3.10-dev'
+        if: matrix.python-version != '3.9' && matrix.python-version != '3.10.0-rc.2'
         run: |
           tox -e py-test
 
@@ -55,7 +55,7 @@ jobs:
           tox -vv -e py-cov
 
       - name: Test nightly Python version with tox
-        if: matrix.python-version == '3.10-dev'
+        if: matrix.python-version == '3.10.0-rc.2'
         # continue-on-error is not ideal since it doesn't give a visible
         # warning, but there doesn't seem to be anything better:
         # https://github.com/actions/toolkit/issues/399