Fix: make gem_search_pattern compatible with rubygems > 3.0

Search pattern for a gem name for rubygems > 3.0 is in format ^gem_name$ instead
of /^gem_name$/ for rubygems < 3.0

/reviewed-by @kirr
/reviewed-on nexedi/rubygemsrecipe!6

rubygems.py

@@ -207,6 +207,7 @@ class Recipe(object):
         }
         recipe = Download(self.buildout, self.name, options)
         recipe.install()
+        self.version = version
 
         current_dir = os.getcwd()
         try:
@@ -292,13 +293,17 @@ class Recipe(object):
             return gem_executable[0]
 
     def get_dependency_list(self, gem_dict, gem_executable):
+        gem_search_pattern = '^' + gem_dict['gemname'].replace('.',r'\.') + '$'
+        if self.version[0] < '3':
+            gem_search_pattern = '/' + gem_search_pattern + '/'
+
         cmd = [
             self.ruby_executable,
             gem_executable,
             'dependency',
             '-rv',
             gem_dict['version'],
-            '/^' + gem_dict['gemname'].replace('.',r'\.') + '$/',
+            gem_search_pattern,
         ]
         cmd_result = self.run(cmd, self._get_env())