Remove Python 2 compatibility shims

The plugins target calibre 5/6 (Python 3) and the Poetry environment
pins Python >=3.8, so the Python 2 fallbacks are dead code. This removes
them throughout both plugins (behaviour on Python 3 is unchanged):

  - Drop all `from __future__` imports.
  - Collapse `if sys.version_info[0] == 2: ... else: ...` blocks to their
    Python 3 branch (ineptpdf, mobidedrm, kindlekey, kgenpids, alfcrypto,
    erdr2pml, ineptepub, obok, and the various unpad() helpers, etc.).
  - Replace `_winreg` import fallbacks with plain `import winreg`, and
    delete the py2-only adobekey_winreg_unicode module (now unreferenced).
  - Drop py2 name shims: `unicode`/`unichr`, `.iteritems()`,
    `from StringIO import StringIO`, `htmlentitydefs` fallback, and the
    Windows CommandLineToArgvW dance in unicode_argv (py3 sys.argv is
    already Unicode on every platform).
  - Remove the "Calibre < 5" (py2) bugfix block from the compat header.

Verified: every .py file in both plugins still byte-compiles.

Scope: the maintained DeDRM_plugin and Obok_plugin only. The archival
standalone scripts under Other_Tools/ are left as historical snapshots.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Obok_plugin/dialogs.py

@@ -1,7 +1,4 @@
 # vim:fileencoding=UTF-8:ts=4:sw=4:sta:et:sts=4:ai
-from __future__ import (unicode_literals, division, absolute_import,
-                        print_function)
-
 __license__   = 'GPL v3'
 __docformat__ = 'restructuredtext en'