skip annotations for version results

Add support for an ebuild to declare a skip annotation for a class of
*version* results. This skip annotation must be declared before any
non-comment non-empty line (most of the times it would be before the
EAPI declaration). Empty lines between the copyright and the annotation
are allowed.
The annotation line must be precise, only one class per line and every
space is required.

For example, it would look like:

  # Copyright 2023 Gentoo Authors
  # Distributed under the terms of the GNU General Public License v2

  # pkgcheck skip: PythonCompatUpdate

  EAPI=8

Resolves: pkgcore#478
Signed-off-by: Arthur Zamarin <arthurzam@gentoo.org>

src/pkgcheck/results.py

@@ -225,6 +225,21 @@ def __init__(self, pkg, **kwargs):
         self.version = pkg.fullver
         self._attr = "version"
 
+        if hasattr(pkg, "lines"):
+            lines = pkg.lines
+        elif ebuild := getattr(pkg, "ebuild", None):
+            with ebuild.text_fileobj() as fileobj:
+                lines = tuple(fileobj)
+        else:
+            lines = ()
+
+        for line in map(str.rstrip, lines):
+            if line and not line.startswith("#"):
+                break  # stop after first non-comment non-empty line
+            elif line == f"# pkgcheck skip: {self.__class__.__name__}":
+                self._filtered = True
+                break
+
     @klass.jit_attr
     def ver_rev(self):
         version, _, revision = self.version.partition("-r")