Use requirements.txt for pip package install_requires

setup.py

@@ -740,15 +740,24 @@ def _picard_packages():
 
 
 this_directory = os.path.abspath(os.path.dirname(__file__))
-with open(os.path.join(this_directory, 'README.md'), encoding='utf-8') as f:
-    long_description = f.read()
+
+
+def _get_description():
+    with open(os.path.join(this_directory, 'README.md'), encoding='utf-8') as f:
+        return f.read()
+
+
+def _get_requirements():
+    with open(os.path.join(this_directory, 'requirements.txt'), encoding='utf-8') as f:
+        return f.readlines()
+
 
 args = {
     'name': PACKAGE_NAME,
     'version': PICARD_VERSION_STR_SHORT,
     'description': 'The next generation MusicBrainz tagger',
     'keywords': 'MusicBrainz metadata tagger picard',
-    'long_description': long_description,
+    'long_description': _get_description(),
     'long_description_content_type': 'text/markdown',
     'url': 'https://picard.musicbrainz.org/',
     'package_dir': {'picard': 'picard'},
@@ -772,7 +781,7 @@ args = {
         'patch_version': picard_patch_version,
     },
     'scripts': ['scripts/' + PACKAGE_NAME],
-    'install_requires': ['PyQt5', 'mutagen', 'python-dateutil', 'fasteners'],
+    'install_requires': _get_requirements(),
     'python_requires': '~=3.6',
     'classifiers': [
         'License :: OSI Approved :: GNU General Public License v2 or later (GPLv2+)',