fix buffered download (#24623)

* Use shutil to do a buffered copy from http response to file

* Fix flake8...

* Somehow flake8 still complains about unrelated files

lib/spack/spack/fetch_strategy.py

@@ -29,11 +29,12 @@
 import re
 import shutil
 import sys
-from typing import Optional, List  # novm
+from typing import List, Optional  # novm
 
-import llnl.util.tty as tty
 import six
 import six.moves.urllib.parse as urllib_parse
+
+import llnl.util.tty as tty
 import spack.config
 import spack.error
 import spack.util.crypto as crypto
@@ -41,9 +42,9 @@
 import spack.util.url as url_util
 import spack.util.web as web_util
 from llnl.util.filesystem import (
-    working_dir, mkdirp, temp_rename, temp_cwd, get_single_file)
+    get_single_file, mkdirp, temp_cwd, temp_rename, working_dir)
 from spack.util.compression import decompressor_for, extension
-from spack.util.executable import which, CommandNotFoundError
+from spack.util.executable import CommandNotFoundError, which
 from spack.util.string import comma_and, quote
 from spack.version import Version, ver
 
@@ -382,12 +383,11 @@ def _fetch_urllib(self, url):
                 os.remove(save_file)
             msg = 'urllib failed to fetch with error {0}'.format(e)
             raise FailedDownloadError(url, msg)
-        _data = response.read()
-        with open(save_file, 'wb') as _open_file:
-            _open_file.write(_data)
-        headers = _data.decode('utf-8', 'ignore')
 
-        self._check_headers(headers)
+        with open(save_file, 'wb') as _open_file:
+            shutil.copyfileobj(response, _open_file)
+
+        self._check_headers(str(headers))
         return None, save_file
 
     @_needs_stage