Stop passing python_exe to pytype_test. (#4488)

pytype hasn't needed the python_exe argument for a while, and getting
rid of it allows some code to be deleted.

tests/pytype_test.py

@@ -13,7 +13,6 @@ will also discover incorrect usage of imported modules.
 import argparse
 import os
 import re
-import subprocess
 import traceback
 from typing import List, Match, Optional, Sequence, Tuple
 
@@ -31,13 +30,10 @@ def main() -> None:
     typeshed_location = args.typeshed_location or os.getcwd()
     subdir_paths = [os.path.join(typeshed_location, d) for d in TYPESHED_SUBDIRS]
     check_subdirs_discoverable(subdir_paths)
-    check_python_exes_runnable(python27_exe_arg=args.python27_exe, python36_exe_arg=args.python36_exe)
     files_to_test = determine_files_to_test(typeshed_location=typeshed_location, paths=args.files or subdir_paths)
     run_all_tests(
         files_to_test=files_to_test,
         typeshed_location=typeshed_location,
-        python27_exe=args.python27_exe,
-        python36_exe=args.python36_exe,
         print_stderr=args.print_stderr,
         dry_run=args.dry_run,
     )
@@ -52,9 +48,6 @@ def create_parser() -> argparse.ArgumentParser:
     parser.add_argument(
         "--print-stderr", action="store_true", default=False, help="Print stderr every time an error is encountered."
     )
-    # We need to invoke python2.7 and 3.6.
-    parser.add_argument("--python27-exe", type=str, default="python2.7", help="Path to a python 2.7 interpreter.")
-    parser.add_argument("--python36-exe", type=str, default="python3.6", help="Path to a python 3.6 interpreter.")
     parser.add_argument(
         "files", metavar="FILE", type=str, nargs="*", help="Files or directories to check. (Default: Check all files.)",
     )
@@ -86,14 +79,13 @@ def load_exclude_list(typeshed_location: str) -> List[str]:
     return skip
 
 
-def run_pytype(*, filename: str, python_version: str, python_exe: str, typeshed_location: str) -> Optional[str]:
+def run_pytype(*, filename: str, python_version: str, typeshed_location: str) -> Optional[str]:
     """Runs pytype, returning the stderr if any."""
     options = pytype_config.Options.create(
         filename,
         module_name=_get_module_name(filename),
         parse_pyi=True,
-        python_version=python_version,
-        python_exe=python_exe)
+        python_version=python_version)
     old_typeshed_home = os.environ.get(TYPESHED_HOME, UNSET)
     os.environ[TYPESHED_HOME] = typeshed_location
     try:
@@ -126,15 +118,6 @@ def _get_module_name(filename: str) -> str:
     return ".".join(_get_relative(filename).split(os.path.sep)[2:]).replace(".pyi", "").replace(".__init__", "")
 
 
-def can_run(exe: str, *, args: List[str]) -> bool:
-    try:
-        subprocess.run([exe] + args, stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
-    except OSError:
-        return False
-    else:
-        return True
-
-
 def _is_version(path: str, version: str) -> bool:
     return any("{}{}{}".format(d, os.path.sep, version) in path for d in TYPESHED_SUBDIRS)
 
@@ -145,19 +128,6 @@ def check_subdirs_discoverable(subdir_paths: List[str]) -> None:
             raise SystemExit("Cannot find typeshed subdir at {} (specify parent dir via --typeshed-location)".format(p))
 
 
-def check_python_exes_runnable(*, python27_exe_arg: str, python36_exe_arg: str) -> None:
-    for exe, version_str in zip([python27_exe_arg, python36_exe_arg], ["27", "36"]):
-        if can_run(exe, args=["--version"]):
-            continue
-        formatted_version = ".".join(list(version_str))
-        script_arg = "--python{}-exe".format(version_str)
-        raise SystemExit(
-            "Cannot run Python {version}. (point to a valid executable via {arg})".format(
-                version=formatted_version, arg=script_arg
-            )
-        )
-
-
 def determine_files_to_test(*, typeshed_location: str, paths: Sequence[str]) -> List[Tuple[str, int]]:
     """Determine all files to test, checking if it's in the exclude list and which Python versions to use.
 
@@ -196,8 +166,6 @@ def run_all_tests(
     *,
     files_to_test: Sequence[Tuple[str, int]],
     typeshed_location: str,
-    python27_exe: str,
-    python36_exe: str,
     print_stderr: bool,
     dry_run: bool
 ) -> None:
@@ -210,7 +178,6 @@ def run_all_tests(
             run_pytype(
                 filename=f,
                 python_version="2.7" if version == 2 else "3.6",
-                python_exe=python27_exe if version == 2 else python36_exe,
                 typeshed_location=typeshed_location,
             )
             if not dry_run