Allow for custom python versions in `python_register_toolchains` (#657)

python/repositories.bzl

diff --git a/python/repositories.bzl b/python/repositories.bzl
index 0968326..3ee418a 100644
--- a/python/repositories.bzl
+++ b/python/repositories.bzl
@@ -183,7 +183,6 @@
         "python_version": attr.string(
             doc = "The Python version.",
             mandatory = True,
-            values = TOOL_VERSIONS.keys() + MINOR_MAPPING.keys(),
         ),
         "release_filename": attr.string(
             doc = "The filename of the interpreter to be downloaded",