Migrate read_file from pkg_resources to importlib.resources

In python 3.9, we can get any resource from a package subdirectory using
files(), but on older versions of python, even where importlib.resources exists,
we need the backport.

README.asciidoc

@@ -131,6 +131,8 @@ The following software and libraries are required to run qutebrowser:
 * http://pygments.org/[pygments]
 * https://github.com/yaml/pyyaml[PyYAML]
 * https://www.attrs.org/[attrs]
+* https://importlib-resources.readthedocs.io/[importlib_resources] (on python
+  3.8 or older)
 
 The following libraries are optional:
 

qutebrowser/utils/utils.py

@@ -42,6 +42,11 @@ from typing import (Any, Callable, IO, Iterator, Optional, Sequence, Tuple, Type
 from PyQt5.QtCore import QUrl, QVersionNumber
 from PyQt5.QtGui import QClipboard, QDesktopServices
 from PyQt5.QtWidgets import QApplication
+# We cannot use the stdlib version on 3.7-3.8 because we need the files() API.
+if sys.version_info >= (3, 9):
+    import importlib.resources as importlib_resources
+else:
+    import importlib_resources
 import pkg_resources
 import yaml
 try:
@@ -216,13 +221,12 @@ def read_file(filename: str, binary: bool = False) -> Any:
             with open(fn, 'r', encoding='utf-8') as f:
                 return f.read()
     else:
-        data = pkg_resources.resource_string(
-            qutebrowser.__name__, filename)
+        p = importlib_resources.files(qutebrowser) / filename
 
         if binary:
-            return data
+            return p.read_bytes()
 
-        return data.decode('UTF-8')
+        return p.read_text()
 
 
 def resource_filename(filename: str) -> str:

qutebrowser/utils/version.py

@@ -263,6 +263,7 @@ def _module_versions() -> Sequence[str]:
         ('pygments', ['__version__']),
         ('yaml', ['__version__']),
         ('attr', ['__version__']),
+        ('importlib_resources', []),
         ('PyQt5.QtWebEngineWidgets', []),
         ('PyQt5.QtWebEngine', ['PYQT_WEBENGINE_VERSION_STR']),
         ('PyQt5.QtWebKitWidgets', []),

setup.py

@@ -71,7 +71,8 @@ try:
         entry_points={'gui_scripts':
                       ['qutebrowser = qutebrowser.qutebrowser:main']},
         zip_safe=True,
-        install_requires=['pypeg2', 'jinja2', 'pygments', 'PyYAML', 'attrs'],
+        install_requires=['pypeg2', 'jinja2', 'pygments', 'PyYAML', 'attrs',
+                          'importlib_resources>=1.1.0; python_version < "3.9"'],
         python_requires='>=3.6',
         name='qutebrowser',
         version=_get_constant('version'),

tests/unit/utils/test_version.py

@@ -561,11 +561,13 @@ class ImportFake:
             ('pygments', True),
             ('yaml', True),
             ('attr', True),
+            ('importlib_resources', True),
             ('PyQt5.QtWebEngineWidgets', True),
             ('PyQt5.QtWebEngine', True),
             ('PyQt5.QtWebKitWidgets', True),
         ])
         self.no_version_attribute = ['sip',
+                                     'importlib_resources',
                                      'PyQt5.QtWebEngineWidgets',
                                      'PyQt5.QtWebKitWidgets',
                                      'PyQt5.QtWebEngine']