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>

DeDRM_plugin/androidkindlekey.py

@@ -51,10 +51,7 @@ STORAGE2 = "map_data_storage.db"
 
 
 def unpad(data, padding=16):
-    if sys.version_info[0] == 2:
-        pad_len = ord(data[-1])
-    else:
-        pad_len = data[-1]
+    pad_len = data[-1]
 
     return data[:-pad_len]