dev: Use types.EllipsisType instead of 'builtins.ellipsis' when we can

The former is preferable as its not a private/internal type¹, but it's
only available on on Python ≥3.10.²

¹ <python/cpython#85976>
² <https://docs.python.org/3/library/types.html#types.EllipsisType>

nextstrain/cli/runner/__init__.py

@@ -1,5 +1,4 @@
 import argparse
-import builtins
 from argparse import ArgumentParser
 from typing import cast, List, Union, TYPE_CHECKING
 # TODO: Use typing.TypeAlias once Python 3.10 is the minimum supported version.
@@ -14,7 +13,7 @@
 from .. import config, env, hostenv
 from ..argparse import DirectoryPath, SKIP_AUTO_DEFAULT_IN_HELP
 from ..errors import UserError
-from ..types import Env, Options, RunnerModule
+from ..types import EllipsisType, Env, Options, RunnerModule
 from ..util import prose_list, runner_name, runner_module, runner_help, warn
 from ..volume import NamedVolume
 
@@ -72,7 +71,7 @@
             % (configured_runner, runner_name(default_runner)))
 
 
-RunnerExec: TypeAlias = List[Union[str, 'builtins.ellipsis']]
+RunnerExec: TypeAlias = List[Union[str, EllipsisType]]
 
 
 def register_runners(parser:  ArgumentParser,

nextstrain/cli/types.py

@@ -4,6 +4,7 @@
 
 import argparse
 import builtins
+import sys
 import urllib.parse
 from pathlib import Path
 from typing import Any, Iterable, List, Mapping, Optional, Tuple, Union
@@ -12,6 +13,12 @@
 from typing_extensions import Protocol, TypeAlias
 from .volume import NamedVolume
 
+# Re-export EllipsisType so we can paper over its absence from older Pythons
+if sys.version_info >= (3, 10):
+    from types import EllipsisType
+else:
+    EllipsisType: TypeAlias = 'builtins.ellipsis'
+
 """
 An immutable mapping of (*name*, *value*) pairs representing a set of
 additional environment variables to overlay on the current environment (e.g.
@@ -33,7 +40,7 @@
 
 RunnerTestResults = List['RunnerTestResult']
 RunnerTestResult  = Tuple[str, 'RunnerTestResultStatus']
-RunnerTestResultStatus: TypeAlias = Union[bool, None, 'builtins.ellipsis']
+RunnerTestResultStatus: TypeAlias = Union[bool, None, EllipsisType]
 
 RunnerUpdateStatus = Optional[bool]