At least the following vanilla Python distributions: 2.7.1rc1, 3.1.2, 3.1.3rc1 
and 3.2a4 are using a new help file naming convention: pythonNNNxxx.chm, where:
NNN - 3 digits of the version, i.e. 271 for version 2.7.1
xxx - trailing string for release type, a.k.a. "rc1", "a4", empty for normal 
releases.

The easiest fix is probably to use the "--pythonXX" switch string as the 
beginning of the filename and look for the first filename to match 
pythonXX*.chm. Should be compatible with old and new distributions.

A more elaborate approach would be to read the "file version" of the 
pythonXX.dll and remove dots from it:
3.2a4 -> 32a4
3.1.3rc1 -> 313rc1
2.7.1rs1 -> 271rc1
But it may break compatibility with older versions as the dlls were already 
using all 3 digits, while the chm only used 2 (major+minor).