Remove unneeded shebangs from test files

The files are not executable, tests should only be run via `pytest`, and
some of them were incorrectly python2.

tests/conftest.py

@@ -1,5 +1,3 @@
-#!/usr/bin/env python3
-
 pytest_plugins = [
     "direnv_project",
     "root",

tests/direnv_project.py

@@ -1,5 +1,3 @@
-#!/usr/bin/env python3
-
 import shutil
 import textwrap
 from dataclasses import dataclass

tests/procs.py

@@ -1,5 +1,3 @@
-#!/usr/bin/env python3
-
 import subprocess
 from pathlib import Path
 from typing import IO, Any, List, Optional, Union

tests/root.py

@@ -1,5 +1,3 @@
-#!/usr/bin/env python3
-
 from pathlib import Path
 
 import pytest

tests/test_gc.py

@@ -1,5 +1,3 @@
-#!/usr/bin/env python2
-
 import subprocess
 import sys
 import unittest

tests/test_use_nix.py

@@ -1,5 +1,3 @@
-#!/usr/bin/env python2
-
 import os
 import subprocess
 import sys