Fix PermissionError when loading .netrc (aio-libs#7237) (aio-libs#7378)

## What do these changes do?

If no NETRC environment variable is provided and the .netrc path cannot
be accessed due to missing permission, a PermissionError was raised
instead of returning None. See issue aio-libs#7237. This PR fixes the issue.

If the changes look good, I can also prepare backports.

## Are there changes in behavior for the user?

If the .netrc cannot be accessed due to a permission problem (and the
`NETRC` environment variable is unset), no `PermissionError` will be
raised. Instead it will be silently ignored.

## Related issue number

Fixes aio-libs#7237

(cherry picked from commit 0d2e43b)

# Conflicts:
#	aiohttp/helpers.py

CHANGES/7237.bugfix

@@ -0,0 +1 @@
+Fixed ``PermissionError`` when .netrc is unreadable due to permissions.

CONTRIBUTORS.txt

@@ -157,6 +157,7 @@ Jake Davis
 Jakob Ackermann
 Jakub Wilk
 Jan Buchar
+Jan Gosmann
 Jarno Elonen
 Jashandeep Sohi
 Jean-Baptiste Estival

aiohttp/helpers.py

@@ -3,6 +3,7 @@
 import asyncio
 import base64
 import binascii
+import contextlib
 import datetime
 import enum
 import functools
@@ -209,8 +210,11 @@ def netrc_from_env() -> Optional[netrc.netrc]:
     except netrc.NetrcParseError as e:
         client_logger.warning("Could not parse .netrc file: %s", e)
     except OSError as e:
+        netrc_exists = False
+        with contextlib.suppress(OSError):
+            netrc_exists = netrc_path.is_file()
         # we couldn't read the file (doesn't exist, permissions, etc.)
-        if netrc_env or netrc_path.is_file():
+        if netrc_env or netrc_exists:
             # only warn if the environment wanted us to load it,
             # or it appears like the default file does actually exist
             client_logger.warning("Could not read .netrc file: %s", e)
@@ -757,7 +761,6 @@ def ceil_timeout(
 
 
 class HeadersMixin:
-
     ATTRS = frozenset(["_content_type", "_content_dict", "_stored_content_type"])
 
     _headers: MultiMapping[str]

tests/test_helpers.py

@@ -6,6 +6,7 @@
 import tempfile
 import weakref
 from math import ceil, modf
+from pathlib import Path
 from unittest import mock
 from urllib.request import getproxies_environment
 
@@ -827,6 +828,26 @@ def test_netrc_from_env(expected_username: str):
     assert netrc_obj.authenticators("example.com")[0] == expected_username
 
 
+@pytest.fixture
+def protected_dir(tmp_path: Path):
+    protected_dir = tmp_path / "protected"
+    protected_dir.mkdir()
+    try:
+        protected_dir.chmod(0o600)
+        yield protected_dir
+    finally:
+        protected_dir.rmdir()
+
+
+def test_netrc_from_home_does_not_raise_if_access_denied(
+    protected_dir: Path, monkeypatch: pytest.MonkeyPatch
+):
+    monkeypatch.setattr(Path, "home", lambda: protected_dir)
+    monkeypatch.delenv("NETRC", raising=False)
+
+    helpers.netrc_from_env()
+
+
 @pytest.mark.parametrize(
     ["netrc_contents", "expected_auth"],
     [