Replace uses of deprecated pkg_resources

Follows
https://importlib-resources.readthedocs.io/en/latest/migration.html to
avoid use of `pkg_resources`, which has been deprecated.

Author: tautschnig

src/cbmc_viewer/report.py

@@ -7,7 +7,7 @@
 import os
 import shutil
 
-import pkg_resources
+from importlib import resources as importlib_resources
 
 from cbmc_viewer import markup_code
 from cbmc_viewer import markup_summary
@@ -37,10 +37,11 @@ def report(config, sources, symbols, results, coverage, traces, properties,
     trace_dir = os.path.join(report_dir, markup_trace.TRACES)
 
     os.makedirs(report_dir, exist_ok=True)
-    shutil.copy(pkg_resources.resource_filename(PACKAGE, VIEWER_CSS),
-                report_dir)
-    shutil.copy(pkg_resources.resource_filename(PACKAGE, VIEWER_JS),
-                report_dir)
+
+    with importlib_resources.path(PACKAGE, VIEWER_CSS) as css_path:
+        shutil.copy(css_path, report_dir)
+    with importlib_resources.path(PACKAGE, VIEWER_JS) as js_path:
+        shutil.copy(js_path, report_dir)
 
     progress("Preparing report summary")
     markup_summary.Summary(

src/cbmc_viewer/templates.py

@@ -6,7 +6,7 @@
 import jinja2
 from jinja2 import select_autoescape
 
-import pkg_resources
+from importlib import resources as importlib_resources
 
 PACKAGE = 'cbmc_viewer'
 TEMPLATES = 'templates'
@@ -24,13 +24,20 @@ def env():
     global ENV
 
     if ENV is None:
-        template_dir = pkg_resources.resource_filename(PACKAGE, TEMPLATES)
-        ENV = jinja2.Environment(
-            loader=jinja2.FileSystemLoader(template_dir),
-            autoescape=select_autoescape(
-                enabled_extensions=('html'),
-                default_for_string=True)
-        )
+        try:
+            ctxmgr =
+            importlib_resources.as_file(importlib_resources.files(PACKAGE) /
+                                        TEMPLATES)
+        except AttributeError:
+            # Python 3.7 and 3.8
+            ctxmgr = importlib_resources.path(PACKAGE, TEMPLATES)
+        with ctxmgr as templates_path:
+            ENV = jinja2.Environment(
+                loader=jinja2.FileSystemLoader(str(templates_path)),
+                autoescape=select_autoescape(
+                    enabled_extensions=('html'),
+                    default_for_string=True)
+            )
     return ENV
 
 def render_summary(summary):