Chokes on broken symlinks

[Enrico Zini]

With the upgrade to bullseye, the packaged bootstrap assets contain broken symlinks. Building a site with it fails:

$ ssite build …
Traceback (most recent call last):
  File "/usr/bin/ssite", line 54, in <module>
    main()
  File "/usr/bin/ssite", line 46, in main
    res = handler.run()
  File "/usr/lib/python3/dist-packages/staticsite/cmd/build.py", line 21, in run
    site = self.load_site()
  File "/usr/lib/python3/dist-packages/staticsite/cmd/command.py", line 61, in load_site
    site.load()
  File "/usr/lib/python3/dist-packages/staticsite/site.py", line 356, in load
    self.scan_content()
  File "/usr/lib/python3/dist-packages/staticsite/site.py", line 318, in scan_content
    self.theme.scan_assets()
  File "/usr/lib/python3/dist-packages/staticsite/theme.py", line 305, in scan_assets
    self.site.scan_tree(
  File "/usr/lib/python3/dist-packages/staticsite/site.py", line 332, in scan_tree
    root.scan(dir_fd)
  File "/usr/lib/python3/dist-packages/staticsite/contents.py", line 24, in wrapper
    return f(self, dir_fd)
  File "/usr/lib/python3/dist-packages/staticsite/contents.py", line 330, in scan
    subdir.scan(subdir_fd)
  File "/usr/lib/python3/dist-packages/staticsite/contents.py", line 24, in wrapper
    return f(self, dir_fd)
  File "/usr/lib/python3/dist-packages/staticsite/contents.py", line 316, in scan
    self.files[entry.name] = file.File.from_dir_entry(self.src, entry)
  File "/usr/lib/python3/dist-packages/staticsite/file.py", line 36, in from_dir_entry
    stat=entry.stat())
FileNotFoundError: [Errno 2] No such file or directory: 'bootstrap-grid.css.map'

We need handling of broken symlinks :/