Implement download_file as contextmanager

It allows us to remove the disable of pylint consider-using-with
warning and does not require any manual file closing.

Signed-off-by: Teodora Sechkova <tsechkova@vmware.com>

Author: sechkova

tuf/ngclient/_internal/download.py

@@ -24,6 +24,7 @@
 """
 import logging
 import tempfile
+from contextlib import contextmanager
 from urllib import parse
 
 from tuf import exceptions
@@ -46,6 +47,7 @@ def __init__(self, fetcher):
 
         self._fetcher = fetcher
 
+    @contextmanager
     def download_file(self, url, required_length):
         """Opens a connection to 'url' and downloads the content
         up to 'required_length'.
@@ -71,33 +73,20 @@ def download_file(self, url, required_length):
         url = parse.unquote(url).replace("\\", "/")
         logger.debug("Downloading: %s", url)
 
+        number_of_bytes_received = 0
         # This is the temporary file that we will return to contain the
         # contents of the downloaded file.
-        temp_file = (
-            tempfile.TemporaryFile()
-        )  # pylint: disable=consider-using-with
-
-        number_of_bytes_received = 0
-
-        try:
+        with tempfile.TemporaryFile() as temp_file:
             chunks = self._fetcher.fetch(url, required_length)
             for chunk in chunks:
                 temp_file.write(chunk)
                 number_of_bytes_received += len(chunk)
-
             if number_of_bytes_received > required_length:
                 raise exceptions.DownloadLengthMismatchError(
                     required_length, number_of_bytes_received
                 )
-
-        except Exception:
-            # Close 'temp_file'.  Any written data is lost.
-            temp_file.close()
-            raise
-
-        else:
             temp_file.seek(0)
-            return temp_file
+            yield temp_file
 
     def download_bytes(self, url, required_length):
         """Download bytes from given url