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/test_errors.py

@@ -0,0 +1,30 @@
+"""Test suite for ``cheroot.errors``."""
+
+import pytest
+
+from cheroot import errors
+
+from .._compat import IS_LINUX, IS_MACOS, IS_WINDOWS  # noqa: WPS130
+
+
+@pytest.mark.parametrize(
+    ('err_names', 'err_nums'),
+    (
+        (('', 'some-nonsense-name'), []),
+        (
+            (
+                'EPROTOTYPE', 'EAGAIN', 'EWOULDBLOCK',
+                'WSAEWOULDBLOCK', 'EPIPE',
+            ),
+            (91, 11, 32) if IS_LINUX else
+            (32, 35, 41) if IS_MACOS else
+            (32, 10041, 11, 10035) if IS_WINDOWS else
+            (),
+        ),
+    ),
+)
+def test_plat_specific_errors(err_names, err_nums):
+    """Test that ``plat_specific_errors`` gets correct error numbers list."""
+    actual_err_nums = errors.plat_specific_errors(*err_names)
+    assert len(actual_err_nums) == len(err_nums)
+    assert sorted(actual_err_nums) == sorted(err_nums)