fix(python): correctly order the toolchains

Since toolchain matching is done by matching the first target that
matches target settings, the `minor_mapping` config setting is special,
because e.g. all `3.11.X` toolchains match the `python_version = "3.11"`
setting.

This just reshuffles the list so that we have toolchains that are in the
`minor_mapping` before the rest.

Fixes bazel-contrib#2685

Author: aignas

python/private/python.bzl

@@ -243,6 +243,18 @@ def parse_modules(*, module_ctx, _fail = fail):
     if len(toolchains) > _MAX_NUM_TOOLCHAINS:
         fail("more than {} python versions are not supported".format(_MAX_NUM_TOOLCHAINS))
 
+    # sort the toolchains so that the toolchain versions that are in the
+    # `minor_mapping` are coming first. This ensures that `python_version =
+    # "3.X"` transitions work as expected.
+    minor_version_toolchains = []
+    other_toolchains = []
+    for t in toolchains:
+        if t.python_version in config.minor_mapping:
+            minor_version_toolchains.append(t)
+        else:
+            other_toolchains.append(t)
+    toolchains = minor_version_toolchains + other_toolchains
+
     return struct(
         config = config,
         debug_info = debug_info,