PEP 594 has cgi module scheduled for deprecation in Python 3.8 (#1649)

* PEP 594 has cgi module scheduled for deprecation in Python 3.8. Reimplement
cgi.parse_header in Sanic. The new implementation is much faster than either
cgi.parse_header or equivalent werkzeug.parse_options_header, and unlike the
two, handles also quoted values with semicolons or \" in them.

* Fix string escape.

* Useless linter complaints.

* More linter issues

* Add return type hint.

* Do not support quoted-pair escapes.

- Improved documentation and renamed the function more aptly as it only seems
  to apply to content-type and content-disposition headers.

* Unquote filenames also in normal mode.

* Add tests for headers. Adapted from CPython parse_header tests with changes on the final test.

* Linter

* Revert "Unquote filenames also in normal mode."

This reverts commit bf0d502bcd.

* Improved parse_content_header and added tests with Firefox and Chrome.

- Unescaping of quotes moved to parse_content_header because it affects all fields,
  not just filenames.
- It is impossible to handle all cases correctly but the current heuristics should
  suffice well for typical cases and beyond.
- Added comparisons with cgi.parse_header and werkzeug.parse_options_header.

* Updated comments as well.

tests/test_headers.py

@@ -0,0 +1,57 @@
+import pytest
+
+from sanic import headers
+
+
+@pytest.mark.parametrize(
+    "input, expected",
+    [
+        ("text/plain", ("text/plain", {})),
+        ("text/vnd.just.made.this.up ; ", ("text/vnd.just.made.this.up", {})),
+        ("text/plain;charset=us-ascii", ("text/plain", {"charset": "us-ascii"})),
+        ('text/plain ; charset="us-ascii"', ("text/plain", {"charset": "us-ascii"})),
+        (
+            'text/plain ; charset="us-ascii"; another=opt',
+            ("text/plain", {"charset": "us-ascii", "another": "opt"})
+        ),
+        (
+            'attachment; filename="silly.txt"',
+            ("attachment", {"filename": "silly.txt"})
+        ),
+        (
+            'attachment; filename="strange;name"',
+            ("attachment", {"filename": "strange;name"})
+        ),
+        (
+            'attachment; filename="strange;name";size=123;',
+            ("attachment", {"filename": "strange;name", "size": "123"})
+        ),
+        (
+            'form-data; name="files"; filename="fo\\"o;bar\\"',
+            ('form-data', {'name': 'files', 'filename': 'fo"o;bar\\'})
+            # cgi.parse_header:
+            # ('form-data', {'name': 'files', 'filename': 'fo"o;bar\\'})
+            # werkzeug.parse_options_header:
+            # ('form-data', {'name': 'files', 'filename': '"fo\\"o', 'bar\\"': None})
+        ),
+        # <input type=file name="foo&quot;;bar\"> with Unicode filename!
+        (
+            # Chrome:
+            # Content-Disposition: form-data; name="foo%22;bar\"; filename="😀"
+            'form-data; name="foo%22;bar\\"; filename="😀"',
+            ('form-data', {'name': 'foo";bar\\', 'filename': '😀'})
+            # cgi: ('form-data', {'name': 'foo%22;bar"; filename="😀'})
+            # werkzeug: ('form-data', {'name': 'foo%22;bar"; filename='})
+        ),
+        (
+            # Firefox:
+            # Content-Disposition: form-data; name="foo\";bar\"; filename="😀"
+            'form-data; name="foo\\";bar\\"; filename="😀"',
+            ('form-data', {'name': 'foo";bar\\', 'filename': '😀'})
+            # cgi: ('form-data', {'name': 'foo";bar"; filename="😀'})
+            # werkzeug: ('form-data', {'name': 'foo";bar"; filename='})
+        ),
+    ]
+)
+def test_parse_headers(input, expected):
+    assert headers.parse_content_header(input) == expected