Stop attempting to read @@//:.python-version

The idea to read .python-version in the root module was problematic
for two reasons. For one thing, if there is a .python-version file in
the root module, but no BUILD or BUILD.bazel file on the top level, we
produce an error that we can't catch. This would be a breaking change,
requiring users to add a BUILD.bazel file. Also, if the user has a
.python-version file, and also sets is_default on a toolchain, we
would fail the build (or, if we didn't, we'd change the default Python
version against the users explicit instructions). That would also be a
breaking change.

Removing the @@//:.python-version logic means the user needs to opt in
by saying python_version_file="@@//.python-version". That's not
necessarily a bad thing, as being explicit allows anyone to grep the
repo for the label and find out what cares about that file (as pointed
out by @fmeum on Slack; good point).

Author: vonschultz

python/private/python.bzl

@@ -118,13 +118,6 @@ def parse_modules(*, module_ctx, _fail = fail):
                     default_python_version_env,
                     default_python_version,
                 )
-    if not default_python_version:
-        fallback_python_version_file = module_ctx.path(Label("@@//:.python-version"))
-        if fallback_python_version_file.exists:
-            default_python_version = module_ctx.read(
-                fallback_python_version_file,
-                watch = "yes",
-            ).strip()
 
     seen_versions = {}
     for mod in module_ctx.modules:
@@ -156,8 +149,7 @@ def parse_modules(*, module_ctx, _fail = fail):
                     is_default = default_python_version == toolchain_version
                     if toolchain_attr.is_default and not is_default:
                         fail("The 'is_default' attribute doesn't work if you set " +
-                             "the default Python version with the `defaults` tag " +
-                             "or the '.python-version' file.")
+                             "the default Python version with the `defaults` tag.")
                 else:
                     is_default = toolchain_attr.is_default
 

tests/python/python_tests.bzl

@@ -364,33 +364,6 @@ def _test_default_from_defaults_file(env):
 
 _tests.append(_test_default_from_defaults_file)
 
-def _test_default_from_defaults_implicit_file(env):
-    py = parse_modules(
-        module_ctx = _mock_mctx(
-            _mod(
-                name = "my_root_module",
-                defaults = [],
-                toolchain = [_toolchain("3.10"), _toolchain("3.11"), _toolchain("3.12")],
-                is_root = True,
-            ),
-            mocked_files = {Label("@@//:.python-version"): "3.12\n"},
-        ),
-    )
-
-    env.expect.that_str(py.default_python_version).equals("3.12")
-
-    want_toolchains = [
-        struct(
-            name = "python_3_" + minor_version,
-            python_version = "3." + minor_version,
-            register_coverage_tool = False,
-        )
-        for minor_version in ["10", "11", "12"]
-    ]
-    env.expect.that_collection(py.toolchains).contains_exactly(want_toolchains)
-
-_tests.append(_test_default_from_defaults_implicit_file)
-
 def _test_first_occurance_of_the_toolchain_wins(env):
     py = parse_modules(
         module_ctx = _mock_mctx(