Implement proper symlink support in ReadTarFS

[althonos]

Type of changes

• Bug fix
• New feature

Checklist

• I've run the latest black with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• I've added tests for new code.
• I accept that @willmcgugan may be pedantic in the code review.

Description

The current version of ReadTarFS does not support symlinks properly where Tar files do. This PR changes the following:

OSFS like behaviour for Info

• Info.is_dir is True for symlinks that map to existing directories.
• ReadTarFS.isdir(path) returns True if path is a symlink that maps to an existing directory.
• ReadTarFS.isfile(path) returns True if the path is a symlink that maps to an existing file.
• ReadTarFs.islink(path) return True if the path is a symlink (that can also be dangling).

Transparent handling of symlinks

• ReadTarFS.openbin(path) also works if path is a symlink that maps to an existing file.
• ReadTarFS.listdir(path) also works if path is a symlink that maps to an existing file.

Closes #409 and partially addresses #425.

[althonos]

Moving this to milestone v2.5.0 because this is a fundamental change.