Change version detection mechanism for Python OpenCV

[psyhtest]

For recent OpenCV versions, it is preferable to parse the package version from version.py. Older versions can still be supported with a simple workaround described in README.

[gfursin]

It does not seem to be backward compatible. The CK policy is to keep packages backward compatible to ensure reproducibility and add extra functionality in the same package only if it backwards compatible or add a new package. I would suggest either to add a new package or to update the old one in a backward compatible way. Thanks!