diff --git a/amodem/__main__.py b/amodem/__main__.py
index 352c4ab..8a9a717 100644
--- a/amodem/__main__.py
+++ b/amodem/__main__.py
@@ -15,11 +15,6 @@ from . import main
 from .config import bitrates
 
 
-# Python 3 has `buffer` attribute for byte-based I/O
-_stdin = getattr(sys.stdin, 'buffer', sys.stdin)
-_stdout = getattr(sys.stdout, 'buffer', sys.stdout)
-
-
 try:
     import argcomplete
 except ImportError:
@@ -83,9 +78,9 @@ def FileType(mode, interface_factory=None):
 
         if fname == '-':
             if 'r' in mode:
-                return _stdin
+                return sys.stdin.buffer
             if 'w' in mode:
-                return _stdout
+                return sys.stdout.buffer
 
         return open(fname, mode)
 
diff --git a/amodem/framing.py b/amodem/framing.py
index f510532..946e4b9 100644
--- a/amodem/framing.py
+++ b/amodem/framing.py
@@ -10,8 +10,7 @@ log = logging.getLogger(__name__)
 
 
 def _checksum_func(x):
-    ''' The result will be unsigned on Python 2/3. '''
-    return binascii.crc32(bytes(x)) & 0xFFFFFFFF
+    return binascii.crc32(bytes(x))
 
 
 class Checksum:
diff --git a/setup.py b/setup.py
index 49ef2f9..d071558 100644
--- a/setup.py
+++ b/setup.py
@@ -32,10 +32,10 @@ setup(
         'Intended Audience :: Information Technology',
         'License :: OSI Approved :: MIT License',
         'Operating System :: POSIX',
-        'Programming Language :: Python :: 3.5',
         'Programming Language :: Python :: 3.6',
         'Programming Language :: Python :: 3.7',
         'Programming Language :: Python :: 3.8',
+        'Programming Language :: Python :: 3.9',
         'Topic :: Software Development :: Libraries :: Python Modules',
         'Topic :: System :: Networking',
         'Topic :: Communications',