Add Python 3.10b4 tests on Ubuntu.

.github/workflows/tests-ubuntu.yml

@@ -45,6 +45,14 @@ jobs:
           env:
             TOXENV: asyncio
 
+        # 3.10-pre
+        - python-version: "3.10.0-beta.4"
+          env:
+            TOXENV: py
+        - python-version: "3.10.0-beta.4"
+          env:
+            TOXENV: asyncio
+
     steps:
     - uses: actions/checkout@v2
 
@@ -54,7 +62,7 @@ jobs:
         python-version: ${{ matrix.python-version }}
 
     - name: Install system libraries
-      if: matrix.python-version == 'pypy3' || contains(matrix.env.TOXENV, 'pinned')
+      if: matrix.python-version == 'pypy3' || contains(matrix.env.TOXENV, 'pinned') || matrix.python-version == '3.10.0-beta.4'
       run: |
         sudo apt-get update
         # libxml2 2.9.12 from ondrej/php PPA breaks lxml so we pin it to the bionic-updates repo version