set pysimplegui as optional dependency

setup.py

@@ -3,14 +3,14 @@ from setuptools import setup
 import os
 import re
 
-shortdesc = "Signal Metadata Format Specification"
-longdesc = """
+shortdesc = 'Signal Metadata Format Specification'
+longdesc = '''
 The Signal Metadata Format (SigMF) specifies a way to describe
 sets of recorded digital signal samples with metadata written in JSON.
 SigMF can be used to describe general information about a collection
 of samples, the characteristics of the system that generated the
 samples, and features of the signal itself.
-"""
+'''
 
 with open(os.path.join('sigmf', '__init__.py')) as handle:
     version = re.search(r'__version__\s*=\s*[\'"]([^\'"]*)[\'"]', handle.read()).group(1)
@@ -29,17 +29,17 @@ setup(
         'Programming Language :: Python :: 3.7',
         'Programming Language :: Python :: 3.8',
     ],
-    entry_points = {
+    entry_points={
         'console_scripts': ['sigmf_validate=sigmf.validate:main']
     },
     packages=['sigmf'],
-    package_data = {
+    package_data={
         'sigmf': ['*.json'],
     },
-    install_requires=[
-        'numpy',
-        'pysimplegui==4.0.0'
-    ],
+    install_requires=['numpy'],
+    extras_require={
+        'gui': 'pysimplegui==4.0.0'
+    },
     setup_requires=['pytest-runner'],
     tests_require=['pytest>3'],
     zip_safe=False