handlers: Close file streams explicitly

No harm in closing these streams explicitly. Best case
scenario, this prevents descriptors leaks, worse case
scenario, it reduces the amount of  messages like the
following during tests:

ResourceWarning: unclosed file

camelot/handlers.py

@@ -70,7 +70,8 @@ class PDFHandler(object):
         if pages == "1":
             page_numbers.append({"start": 1, "end": 1})
         else:
-            infile = PdfFileReader(open(filepath, "rb"), strict=False)
+            instream = open(filepath, "rb")
+            infile = PdfFileReader(instream, strict=False)
             if infile.isEncrypted:
                 infile.decrypt(self.password)
             if pages == "all":
@@ -84,6 +85,7 @@ class PDFHandler(object):
                         page_numbers.append({"start": int(a), "end": int(b)})
                     else:
                         page_numbers.append({"start": int(r), "end": int(r)})
+            instream.close()
         P = []
         for p in page_numbers:
             P.extend(range(p["start"], p["end"] + 1))
@@ -122,7 +124,8 @@ class PDFHandler(object):
             if rotation != "":
                 fpath_new = "".join([froot.replace("page", "p"), "_rotated", fext])
                 os.rename(fpath, fpath_new)
-                infile = PdfFileReader(open(fpath_new, "rb"), strict=False)
+                instream = open(fpath_new, "rb")
+                infile = PdfFileReader(instream, strict=False)
                 if infile.isEncrypted:
                     infile.decrypt(self.password)
                 outfile = PdfFileWriter()
@@ -134,6 +137,7 @@ class PDFHandler(object):
                 outfile.addPage(p)
                 with open(fpath, "wb") as f:
                     outfile.write(f)
+                instream.close()
 
     def parse(
         self, flavor="lattice", suppress_stdout=False, layout_kwargs={}, **kwargs