Update tox.ini config

tox.ini

@@ -1,43 +1,37 @@
 [tox]
 envlist = 
-    ; qt latest
-    py{37,38}-{pyqt5,pyside2}_latest
+    ; qt 5.15.x
+    py{37,38,39}-{pyqt5,pyside2}_515
 
-    ; qt 5.12.x (LTS) 
-    py{36,37}-{pyqt5,pyside2}_512
+    ; qt 5.12.x
+    py{37}-{pyqt5,pyside2}_512
+    ; py38-pyside2_512 doesn't work due to PYSIDE-1140
+    py38-pyqt5_512 
 
-    ; qt 5.9.7 (LTS)
-    py36-{pyqt5,pyside2}_59_conda
+    ; qt 6
+    py{37,38,39}-{pyqt6,pyside6}
 
 [base]
 deps =
     pytest
+    pytest-xdist
     numpy
     scipy
     pyopengl
-    flake8
-    coverage
+    h5py
 
 [testenv]
-passenv = DISPLAY XAUTHORITY
+passenv = DISPLAY XAUTHORITY, PYTHON_VERSION
 setenv = PYTHONWARNINGS=ignore:DEPRECATION::pip._internal.cli.base_command
 deps=
     {[base]deps}
-    pytest-cov
-    h5py
-    pyside2_512: pyside2>=5.12,<5.13
-    pyqt5_512: pyqt5>=5.12,<5.13
-    pyside2_latest: pyside2
-    pyqt5_latest: pyqt5
-
-conda_deps=
-    pyside2_59_conda: pyside2=5.9
-    pyqt5_59_conda: pyqt=5.9
-
-conda_channels=
-    conda-forge
-    free
+    pyside2_512: pyside2==5.12.6
+    pyqt5_512: pyqt5==5.12.3
+    pyside2_515: pyside2
+    pyqt5_515: pyqt5
+    pyqt6: pyqt6
+    pyside6: pyside6
 
 commands=
     python -c "import pyqtgraph as pg; pg.systemInfo()"
-    pytest {posargs:} 
+    pytest -n auto {posargs:}