BlockPolicy: Compile regex once rather than once per migration item

Regex compilation is often rather expensive and in this case, we can
do it once instad of once per migration item.

Signed-off-by: Niels Thykier <niels@thykier.net>

britney2/policies/policy.py

@@ -929,6 +929,8 @@ class BuildDependsPolicy(BasePolicy):
 
 class BlockPolicy(BasePolicy):
 
+    BLOCK_HINT_REGEX = re.compile('^(un)?(block-?.*)$')
+
     def __init__(self, options, suite_info):
         super().__init__('block', options, suite_info,
                          {SuiteClass.PRIMARY_SOURCE_SUITE, SuiteClass.ADDITIONAL_SOURCE_SUITE})
@@ -966,7 +968,7 @@ class BlockPolicy(BasePolicy):
 
         shints = self.hints.search(package=src)
         mismatches = False
-        r = re.compile('^(un)?(block-?.*)$')
+        r = self.BLOCK_HINT_REGEX
         for hint in shints:
             m = r.match(hint.type)
             if m: