Use utf8 c_string_encoding

Switch the Cython c_string_encoding directive from "ascii" to "utf8" so
implicit char* <-> str conversions (e.g. av.Dictionary options, FFmpeg
error strings) no longer raise UnicodeDecode/EncodeError on non-ASCII
data.

Author: WyattBlue

setup.py

@@ -163,7 +163,7 @@ def parse_cflags(raw_flags):
 
 compiler_directives = {
     "c_string_type": "str",
-    "c_string_encoding": "ascii",
+    "c_string_encoding": "utf8",
     "embedsignature": True,
     "binding": False,
     "language_level": 3,

tests/test_dictionary.py

@@ -15,3 +15,12 @@ def test_dictionary() -> None:
     assert d.pop("key") == "value"
     pytest.raises(KeyError, d.pop, "key")
     assert len(d) == 0
+
+
+def test_dictionary_non_ascii() -> None:
+    d = Dictionary()
+    d["café"] = "naïve 日本語 🎵"
+
+    assert d["café"] == "naïve 日本語 🎵"
+    assert "café" in d
+    assert list(d) == ["café"]