Cosmetic: use SPDX license header (!1877)

While at it, also remove unnecessary "#!/usr/bin/env python3" in files
that only get imported, and adjust other empty/comment lines in the
beginnings of the files for consistency.

This makes files easier to read, and makes the pmbootstrap codebase more
consistent with the build.postmarketos.org codebase.

pmb/helpers/http.py

@@ -1,21 +1,5 @@
-"""
-Copyright 2020 Oliver Smith
-
-This file is part of pmbootstrap.
-
-pmbootstrap is free software: you can redistribute it and/or modify
-it under the terms of the GNU General Public License as published by
-the Free Software Foundation, either version 3 of the License, or
-(at your option) any later version.
-
-pmbootstrap is distributed in the hope that it will be useful,
-but WITHOUT ANY WARRANTY; without even the implied warranty of
-MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
-GNU General Public License for more details.
-
-You should have received a copy of the GNU General Public License
-along with pmbootstrap.  If not, see <http://www.gnu.org/licenses/>.
-"""
+# Copyright 2020 Oliver Smith
+# SPDX-License-Identifier: GPL-3.0-or-later
 import hashlib
 import json
 import logging