Initial python3 changes

Mostly just updating libraries, removing string encoding/decoding,
fixing some edge cases. No new functionality was added in this
commit.

lib/cheroot/test/_pytest_plugin.py

@@ -0,0 +1,50 @@
+"""Local pytest plugin.
+
+Contains hooks, which are tightly bound to the Cheroot framework
+itself, useless for end-users' app testing.
+"""
+
+from __future__ import absolute_import, division, print_function
+__metaclass__ = type
+
+import pytest
+
+
+pytest_version = tuple(map(int, pytest.__version__.split('.')))
+
+
+def pytest_load_initial_conftests(early_config, parser, args):
+    """Drop unfilterable warning ignores."""
+    if pytest_version < (6, 2, 0):
+        return
+
+    # pytest>=6.2.0 under Python 3.8:
+    # Refs:
+    # * https://docs.pytest.org/en/stable/usage.html#unraisable
+    # * https://github.com/pytest-dev/pytest/issues/5299
+    early_config._inicache['filterwarnings'].extend((
+        'ignore:Exception in thread CP Server Thread-:'
+        'pytest.PytestUnhandledThreadExceptionWarning:_pytest.threadexception',
+        'ignore:Exception in thread Thread-:'
+        'pytest.PytestUnhandledThreadExceptionWarning:_pytest.threadexception',
+        'ignore:Exception ignored in. '
+        '<socket.socket fd=-1, family=AddressFamily.AF_INET, '
+        'type=SocketKind.SOCK_STREAM, proto=.:'
+        'pytest.PytestUnraisableExceptionWarning:_pytest.unraisableexception',
+        'ignore:Exception ignored in. '
+        '<socket.socket fd=-1, family=AddressFamily.AF_INET6, '
+        'type=SocketKind.SOCK_STREAM, proto=.:'
+        'pytest.PytestUnraisableExceptionWarning:_pytest.unraisableexception',
+        'ignore:Exception ignored in. '
+        '<socket.socket fd=-1, family=AF_INET, '
+        'type=SocketKind.SOCK_STREAM, proto=.:'
+        'pytest.PytestUnraisableExceptionWarning:_pytest.unraisableexception',
+        'ignore:Exception ignored in. '
+        '<socket.socket fd=-1, family=AF_INET6, '
+        'type=SocketKind.SOCK_STREAM, proto=.:'
+        'pytest.PytestUnraisableExceptionWarning:_pytest.unraisableexception',
+        'ignore:Exception ignored in. '
+        '<ssl.SSLSocket fd=-1, family=AddressFamily.AF_UNIX, '
+        'type=SocketKind.SOCK_STREAM, proto=.:'
+        'pytest.PytestUnraisableExceptionWarning:_pytest.unraisableexception',
+    ))