net/nsscache: Adjust RUN_DEPENDS for Python 3.8.7 and onward

• Since Python 3.8.7 (and also 3.9.2), .so files get an extension, e.g. ".cpython-38.so" if built with Python 3.8.
  
  Thus check for the presence of the package from ftp/py-pycurl instead of the actual .so file itself.

PR: 254424
Approved by: kbowling (maintainer)