Drop support for running without setuptools.

setup.py

@@ -1,20 +1,12 @@
 #!/usr/bin/python3
 # encoding: utf-8
 # Setup file for dulwich
-# Copyright (C) 2008-2016 Jelmer Vernooij <jelmer@jelmer.uk>
-
-try:
-    from setuptools import setup, Extension
-except ImportError:
-    from distutils.core import setup, Extension
-    has_setuptools = False
-else:
-    has_setuptools = True
-from distutils.core import Distribution
+# Copyright (C) 2008-2022 Jelmer Vernooij <jelmer@jelmer.uk>
+
+from setuptools import setup, Extension, Distribution
 import io
 import os
 import sys
-from typing import Dict, Any
 
 
 if sys.version_info < (3, 6):
@@ -71,26 +63,7 @@ ext_modules = [
     Extension('dulwich._diff_tree', ['dulwich/_diff_tree.c']),
 ]
 
-setup_kwargs = {}  # type: Dict[str, Any]
 scripts = ['bin/dul-receive-pack', 'bin/dul-upload-pack']
-if has_setuptools:
-    setup_kwargs['extras_require'] = {
-        'fastimport': ['fastimport'],
-        'https': ['urllib3>=1.24.1'],
-        'pgp': ['gpg'],
-        'paramiko': ['paramiko'],
-    }
-    setup_kwargs['install_requires'] = ['urllib3>=1.24.1', 'certifi']
-    setup_kwargs['include_package_data'] = True
-    setup_kwargs['test_suite'] = 'dulwich.tests.test_suite'
-    setup_kwargs['tests_require'] = tests_require
-    setup_kwargs['entry_points'] = {
-        "console_scripts": [
-            "dulwich=dulwich.cli:main",
-        ]}
-    setup_kwargs['python_requires'] = '>=3.6'
-else:
-    scripts.append('bin/dulwich')
 
 
 with io.open(os.path.join(os.path.dirname(__file__), "README.rst"),
@@ -117,7 +90,15 @@ setup(name='dulwich',
       scripts=scripts,
       ext_modules=ext_modules,
       zip_safe=False,
-      distclass=DulwichDistribution,
+      distclass=DulwichDistribution,  # type: ignore
+      install_requires=['urllib3>=1.24.1', 'certifi'],
+      include_package_data=True,
+      test_suite='dulwich.tests.test_suite',
+      tests_require=tests_require,
+      entry_points={
+          "console_scripts": ["dulwich=dulwich.cli:main"]
+      },
+      python_requires='>=3.6',
       classifiers=[
           'Development Status :: 4 - Beta',
           'License :: OSI Approved :: Apache Software License',
@@ -133,5 +114,9 @@ setup(name='dulwich',
           'Operating System :: Microsoft :: Windows',
           'Topic :: Software Development :: Version Control',
       ],
-      **setup_kwargs
-      )
+      extras_require={
+          'fastimport': ['fastimport'],
+          'https': ['urllib3>=1.24.1'],
+          'pgp': ['gpg'],
+          'paramiko': ['paramiko'],
+      })