Fix crash when printing while capsysbinary is active

Previously, writing to sys.stdout/stderr in text-mode (e.g.
`print('foo')`) while a `capsysbinary` fixture is active, would crash
with:

    /usr/lib/python3.7/contextlib.py:119: in __exit__
        next(self.gen)
    E   TypeError: write() argument must be str, not bytes

This is due to some confusion in the types. The relevant functions are
`snap()` and `writeorg()`. The function `snap()` returns what was
captured, and the return type should be `bytes` for the binary captures
and `str` for the regular ones. The `snap()` return value is eventually
passed to `writeorg()` to be written to the original file, so it's input
type should correspond to `snap()`. But this was incorrect for
`SysCaptureBinary`, which handled it like `str`.

To fix this, be explicit in the `snap()` and `writeorg()`
implementations, also of the other Capture types.

We can't add type annotations yet, because the current inheritance
scheme breaks Liskov Substitution and mypy would complain. To be
refactored later.

Fixes: https://github.com/pytest-dev/pytest/issues/6871
Co-authored-by: Ran Benita (some modifications & commit message)

testing/test_capture.py

@@ -515,18 +515,40 @@ class TestCaptureFixture:
         reprec.assertoutcome(passed=1)
 
     def test_capsysbinary(self, testdir):
-        reprec = testdir.inline_runsource(
-            """\
+        p1 = testdir.makepyfile(
+            r"""
             def test_hello(capsysbinary):
                 import sys
-                # some likely un-decodable bytes
-                sys.stdout.buffer.write(b'\\xfe\\x98\\x20')
+
+                sys.stdout.buffer.write(b'hello')
+
+                # Some likely un-decodable bytes.
+                sys.stdout.buffer.write(b'\xfe\x98\x20')
+
+                sys.stdout.buffer.flush()
+
+                # Ensure writing in text mode still works and is captured.
+                # https://github.com/pytest-dev/pytest/issues/6871
+                print("world", flush=True)
+
                 out, err = capsysbinary.readouterr()
-                assert out == b'\\xfe\\x98\\x20'
+                assert out == b'hello\xfe\x98\x20world\n'
                 assert err == b''
+
+                print("stdout after")
+                print("stderr after", file=sys.stderr)
             """
         )
-        reprec.assertoutcome(passed=1)
+        result = testdir.runpytest(str(p1), "-rA")
+        result.stdout.fnmatch_lines(
+            [
+                "*- Captured stdout call -*",
+                "stdout after",
+                "*- Captured stderr call -*",
+                "stderr after",
+                "*= 1 passed in *",
+            ]
+        )
 
     def test_partial_setup_failure(self, testdir):
         p = testdir.makepyfile(
@@ -890,7 +912,7 @@ class TestFDCapture:
         cap.start()
         tmpfile.write(data1)
         tmpfile.flush()
-        cap.writeorg(data2)
+        cap.writeorg(data2.decode("ascii"))
         scap = cap.snap()
         cap.done()
         assert scap == data1.decode("ascii")