build: accept both python 3 and python 2 This makes building GiNaC a bit easier for users of "old" distributions where python 3 is not installed by default (Ubuntu 16.04), or even not available (CentOS 7). Scripts themselves work just fine with python 2.7 and python 3.x, so only minor changes to CMakeLists.txt are required (autotools scripts already handle python2/3)
Convert shebangs to python3. PEP 394 does not require the python command to be installed.
Convert YAPTU to Python3. It should now work with python2 and python3.
Convert function.pl into C++ (well, almost) source and header. The code is much more readable now, as it's (almost) plain C++ (except simple pythonic `preprocessor' instructions). As a side effect perl is no longer necessary for building GiNaC.