statprof: use context manager when reading source from file

Differential Revision: https://phab.mercurial-scm.org/D6779

mercurial/statprof.py

@@ -236,18 +236,14 b' class CodeSite(object):'
     def getsource(self, length):
         if self.source is None:
             lineno = self.lineno - 1
-            fp = None
             try:
-                fp = open(self.path, 'rb')
+                with open(self.path, 'rb') as fp:
                 for i, line in enumerate(fp):
                     if i == lineno:
                         self.source = line.strip()
                         break
             except:
                 pass
-            finally:
-                if fp:
-                    fp.close()
             if self.source is None:
                 self.source = ''