Add support for python 3.9.

.github/workflows/pythonpackage.yml

@@ -9,7 +9,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        python-version: [3.5, 3.6, 3.7, 3.8, pypy3]
+        python-version: [3.5, 3.6, 3.7, 3.8, 3.9, pypy3]
         exclude:
           # sqlite3 exit handling seems to get in the way
           - os: macos-latest

README.rst

@@ -85,8 +85,8 @@ file and `list of open issues <https://github.com/dulwich/dulwich/issues>`_.
 Supported versions of Python
 ----------------------------
 
-At the moment, Dulwich supports (and is tested on) CPython 3.5, 3.6,
-3.7, 3.8 and Pypy.
+At the moment, Dulwich supports (and is tested on) CPython 3.5 and later and
+Pypy.
 
 The latest release series to support Python 2.x was the 0.19 series. See
 the 0.19 branch in the Dulwich git repository.

setup.py

@@ -124,6 +124,7 @@ setup(name='dulwich',
           'Programming Language :: Python :: 3.6',
           'Programming Language :: Python :: 3.7',
           'Programming Language :: Python :: 3.8',
+          'Programming Language :: Python :: 3.9',
           'Programming Language :: Python :: Implementation :: CPython',
           'Programming Language :: Python :: Implementation :: PyPy',
           'Operating System :: POSIX',