The build instructions on the sigrok Wiki only work for Python2, which is past its end of life date. To build libsigrok with Python bindings for Python3, you need to set PYTHON=python3 when running configure.

The dependency list is also slightly different:

sudo apt-get install git-core gcc g++ make autoconf autoconf-archive \
  automake libtool pkg-config libglib2.0-dev libglibmm-2.4-dev libzip-dev \
  libusb-1.0-0-dev libftdi1-dev check doxygen python3-numpy \
  python3-dev python-gi-dev python3-setuptools swig default-jdk

"python-gi-dev" is not a typo -- the package covers both Python2 and Python3.

Side note: Installing libserialport-dev instead of building your own version as documented on the Wiki seems to work fine.