[mypy] run more tests in parallel

At some point, Github Actions runners started having more cores:
https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners/about-github-hosted-runners#standard-github-hosted-runners-for-public-repositories

.github/workflows/test.yml

@@ -36,53 +36,53 @@ jobs:
           arch: x64
           os: ubuntu-latest
           toxenv: py
-          tox_extra_args: "-n 2"
+          tox_extra_args: "-n 4"
           test_mypyc: true
         - name: Test suite with py38-windows-64
           python: '3.8'
           arch: x64
           os: windows-latest
           toxenv: py38
-          tox_extra_args: "-n 2"
+          tox_extra_args: "-n 4"
         - name: Test suite with py39-ubuntu
           python: '3.9'
           arch: x64
           os: ubuntu-latest
           toxenv: py
-          tox_extra_args: "-n 2"
+          tox_extra_args: "-n 4"
         - name: Test suite with py310-ubuntu
           python: '3.10'
           arch: x64
           os: ubuntu-latest
           toxenv: py
-          tox_extra_args: "-n 2"
+          tox_extra_args: "-n 4"
         - name: Test suite with py311-ubuntu, mypyc-compiled
           python: '3.11'
           arch: x64
           os: ubuntu-latest
           toxenv: py
-          tox_extra_args: "-n 2"
+          tox_extra_args: "-n 4"
           test_mypyc: true
         - name: Test suite with py312-ubuntu, mypyc-compiled
           python: '3.12'
           arch: x64
           os: ubuntu-latest
           toxenv: py
-          tox_extra_args: "-n 2"
+          tox_extra_args: "-n 4"
           test_mypyc: true
 
         - name: mypyc runtime tests with py39-macos
           python: '3.9.18'
           arch: x64
           os: macos-latest
           toxenv: py
-          tox_extra_args: "-n 2 mypyc/test/test_run.py mypyc/test/test_external.py"
+          tox_extra_args: "-n 3 mypyc/test/test_run.py mypyc/test/test_external.py"
         - name: mypyc runtime tests with py38-debug-build-ubuntu
           python: '3.8.17'
           arch: x64
           os: ubuntu-latest
           toxenv: py
-          tox_extra_args: "-n 2 mypyc/test/test_run.py mypyc/test/test_external.py"
+          tox_extra_args: "-n 3 mypyc/test/test_run.py mypyc/test/test_external.py"
           debug_build: true
 
         - name: Type check our own code (py38-ubuntu)
@@ -189,4 +189,4 @@ jobs:
       - name: Setup tox environment
         run: tox run -e py --notest
       - name: Test
-        run: tox run -e py --skip-pkg-install -- -n 2 mypyc/test/
+        run: tox run -e py --skip-pkg-install -- -n 4 mypyc/test/