Use a template for git pre-commit hook

Previously, we forced Flake8 to be installed in whatever Python
environment that the user was using. This allows someone to use Flake8
from a virtual environment, e.g., if you use the tox.ini from this
commit, you can do:

    tox -e venv -- flake8 --install-hook git

And that will allow you to use the Python and Flake8 from
./.tox/venv/bin/python. This means that you can avoid installing
Flake8 globally and still have a working commit hook.

flake8/main/git.py

@@ -9,6 +9,7 @@ import os
 import shutil
 import stat
 import subprocess
+import sys
 import tempfile
 
 from flake8 import defaults
@@ -80,8 +81,10 @@ def install():
             path=pre_commit_file,
         )
 
+    executable = get_executable()
+
     with open(pre_commit_file, 'w') as fd:
-        fd.write(_HOOK_TEMPLATE)
+        fd.write(_HOOK_TEMPLATE.format(executable=executable))
 
     # NOTE(sigmavirus24): The following sets:
     # - read, write, and execute permissions for the owner
@@ -94,6 +97,12 @@ def install():
     return True
 
 
+def get_executable():
+    if sys.executable is not None:
+        return sys.executable
+    return '/usr/bin/env python'
+
+
 def find_git_directory():
     rev_parse = piped_process(['git', 'rev-parse', '--git-dir'])
 
@@ -182,7 +191,7 @@ def config_for(parameter):
     return value.lower() in defaults.TRUTHY_VALUES
 
 
-_HOOK_TEMPLATE = """#!/usr/bin/env python
+_HOOK_TEMPLATE = """#!{executable}
 import os
 import sys