Use SPDX-License-Identifier consistently

This replaces all of the Apache blurbs at the start of each file with an
`# SPDX-License-Identifier: Apache-2.0` string. It also fixes existing
uses to be consistent in capitalization, and to be placed above any
copyright notices.

This removes nearly 3000 lines of boilerplate from the project (only
python files), which no developer cares to look at.

SPDX is in common use, particularly in the Linux kernel, and is the
recommended format for Meson's own `project(license: )` field
diff --git a/skip_ci.py b/skip_ci.py
index 4ade979..4440826 100755
--- a/skip_ci.py
+++ b/skip_ci.py
@@ -1,19 +1,7 @@
 #!/usr/bin/env python3
+# SPDX-License-Identifier: Apache-2.0
 # Copyright 2018 The Meson development team
 
-# Licensed under the Apache License, Version 2.0 (the "License");
-# you may not use this file except in compliance with the License.
-# You may obtain a copy of the License at
-
-#     http://www.apache.org/licenses/LICENSE-2.0
-
-# Unless required by applicable law or agreed to in writing, software
-# distributed under the License is distributed on an "AS IS" BASIS,
-# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-# See the License for the specific language governing permissions and
-# limitations under the License.
-
-
 import argparse
 import os
 import subprocess