ci: disable builds for 3.10.0a4, and enable a nightly 3.10-dev build (#2792)

* Disable builds for 3.10.0a4, and enable a nightly 3.10-dev build

* Fix job name

* Remove deadsnakes job for now

* Add deadsnakes jobs

* There's no deadsnakes 2.7

* Add 3.10 to versions to be discovered by legacy FindPython, and fix debug input to deadsnakes/action

* Try out branch with fix

* Update to deadsnakes/action@v2.1.1

.github/workflows/ci.yml

diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 7f1d493..3fa9c4e 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -22,7 +22,7 @@
         - 3.5
         - 3.6
         - 3.9
-        - 3.10-dev
+        # - 3.10-dev  # Re-enable once 3.10.0a5 is released
         - pypy2
         - pypy3
 
@@ -165,6 +165,49 @@
       run: pytest tests/extra_setuptools
 
 
+  deadsnakes:
+    strategy:
+      fail-fast: false
+      matrix:
+        python:
+        - version: 3.9
+          debug: true
+        - version: 3.10-dev
+          debug: false
+
+    name: "🐍 ${{ matrix.python.version }}${{ matrix.python.debug && ' (debug)' || '' }} • deadsnakes • x64"
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Setup Python ${{ matrix.python.version }} (deadsnakes)
+      uses: deadsnakes/action@v2.1.1
+      with:
+        python-version: ${{ matrix.python.version }}
+        debug: ${{ matrix.python.debug }}
+
+    - name: Prepare env
+      run: python -m pip install -r tests/requirements.txt --prefer-binary
+
+    - name: Configure
+      run: >
+        cmake -S . -B build
+        -DPYBIND11_WERROR=ON
+        -DDOWNLOAD_CATCH=ON
+        -DDOWNLOAD_EIGEN=ON
+        -DCMAKE_CXX_STANDARD=17
+
+    - name: Build
+      run: cmake --build build -j 2
+
+    - name: Python tests
+      run: cmake --build build --target pytest
+
+    - name: C++ tests
+      run: cmake --build build --target cpptest
+
+
   # Testing on clang using the excellent silkeh clang docker images
   clang:
     runs-on: ubuntu-latest

tools/pybind11Tools.cmake

diff --git a/tools/pybind11Tools.cmake b/tools/pybind11Tools.cmake
index cdd1979..aee60db 100644
--- a/tools/pybind11Tools.cmake
+++ b/tools/pybind11Tools.cmake
@@ -31,7 +31,7 @@
 
 # A user can set versions manually too
 set(Python_ADDITIONAL_VERSIONS
-    "3.9;3.8;3.7;3.6;3.5;3.4"
+    "3.10;3.9;3.8;3.7;3.6;3.5;3.4"
     CACHE INTERNAL "")
 
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_LIST_DIR}")