diff --git a/setup.py b/setup.py
index 0e2f53c..4569fe5 100644
--- a/setup.py
+++ b/setup.py
@@ -46,5 +46,5 @@
         'sonata': SONATA_REQUIRES,
     },
     packages=find_packages(),
-    python_requires='>=3.6',
+    python_requires='>=3.7',
 )
diff --git a/tox.ini b/tox.ini
index a9c649f..e4ea3d2 100644
--- a/tox.ini
+++ b/tox.ini
@@ -8,7 +8,7 @@ envlist =
     check-version
     check-packaging
     lint
-    py{37,38}
+    py{37,38,39}
 
 minversion = 3.18.0
 
@@ -16,7 +16,7 @@ minversion = 3.18.0
 ignore_basepython_conflict = true
 
 [testenv]
-basepython=python3.8
+basepython=python3.9
 setenv =
     PIP_INDEX_URL = https://bbpteam.epfl.ch/repository/devpi/simple
 extras = all
diff --git a/voxcell/version.py b/voxcell/version.py
index ef9450b..33ceefa 100644
--- a/voxcell/version.py
+++ b/voxcell/version.py
@@ -1,3 +1,3 @@
 """Voxcell version."""
-VERSION = '3.1.0'
+VERSION = '3.1.1.dev0'
 __version__ = VERSION