Change license headers to shorter version and add spdx identifier

Signed-off-by: Daniel Lim Wee Soong <weesoong.lim@gmail.com>

v2x/lib/argparse_extra.py

diff --git a/v2x/lib/argparse_extra.py b/v2x/lib/argparse_extra.py
index b078f08..d9ef839 100644
--- a/v2x/lib/argparse_extra.py
+++ b/v2x/lib/argparse_extra.py
@@ -2,17 +2,11 @@
 
 # Copyright (C) 2020  The SymbiFlow Authors.
 #
-# Permission to use, copy, modify, and/or distribute this software for any
-# purpose with or without fee is hereby granted, provided that the above
-# copyright notice and this permission notice appear in all copies.
+# Use of this source code is governed by a ISC-style
+# license that can be found in the LICENSE file or at
+# https://opensource.org/licenses/ISC
 #
-# THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
-# WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
-# MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
-# ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
-# WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
-# ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
-# OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+# SPDX-License-Identifier:	ISC
 
 import argparse
 

v2x/lib/asserts.py

diff --git a/v2x/lib/asserts.py b/v2x/lib/asserts.py
index 18bacb0..f823784 100644
--- a/v2x/lib/asserts.py
+++ b/v2x/lib/asserts.py
@@ -2,19 +2,11 @@
 
 # Copyright (C) 2020  The SymbiFlow Authors.
 #
-# Permission to use, copy, modify, and/or distribute this software for any
-# purpose with or without fee is hereby granted, provided that the above
-# copyright notice and this permission notice appear in all copies.
+# Use of this source code is governed by a ISC-style
+# license that can be found in the LICENSE file or at
+# https://opensource.org/licenses/ISC
 #
-# THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
-# WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
-# MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
-# ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
-# WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
-# ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
-# OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
-#
-# SPDX-License-Identifier: ISC
+# SPDX-License-Identifier:	ISC
 
 
 def __safe_call(f, a):

v2x/lib/mux.py

diff --git a/v2x/lib/mux.py b/v2x/lib/mux.py
index bfd74f8..e1a47a4 100644
--- a/v2x/lib/mux.py
+++ b/v2x/lib/mux.py
@@ -2,17 +2,11 @@
 
 # Copyright (C) 2020  The SymbiFlow Authors.
 #
-# Permission to use, copy, modify, and/or distribute this software for any
-# purpose with or without fee is hereby granted, provided that the above
-# copyright notice and this permission notice appear in all copies.
+# Use of this source code is governed by a ISC-style
+# license that can be found in the LICENSE file or at
+# https://opensource.org/licenses/ISC
 #
-# THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
-# WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
-# MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
-# ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
-# WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
-# ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
-# OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+# SPDX-License-Identifier:	ISC
 
 import lxml.etree as ET
 from enum import Enum