actually change the default Python version to 3.10

this default lives too many places!

repo2docker/buildpacks/conda/__init__.py

@@ -141,7 +141,7 @@ class CondaBuildPack(BaseImage):
             ),
         ]
 
-    major_pythons = {"2": "2.7", "3": "3.7"}
+    major_pythons = {"2": "2.7", "3": "3.10"}
 
     def get_build_script_files(self):
         """

tests/venv/default/verify

@@ -2,7 +2,7 @@
 # Verify that the default just provides a py3 environment with jupyter
 import sys
 
-assert sys.version_info[:2] == (3, 7), sys.version
+assert sys.version_info[:2] == (3, 10), sys.version
 import jupyter
 
 with open("/tmp/appendix") as f:

tests/venv/numpy/verify

@@ -1,6 +1,6 @@
 #!/usr/bin/env python
 import sys
 
-assert sys.version_info[:2] == (3, 7)
+assert sys.version_info[:2] == (3, 10)
 
 import numpy