title: Can't build SciPy from source because pybind11 missing
body: <p>I have ssh'ed into my Raspberry Pi 3b+ on my Macbook. I am trying to install SciPy on Python 3.8, but apt-get does not work and installing through pip freezes my Raspberry Pi. I have even tried increasing swap size, but it still freezes. As a result, I am trying to build SciPy 1.6 from source, but when I run <code>sudo python3 setup.py build</code> and <code>sudo python3 setup.py install</code>, it returns <code>Error: 'pybind11' must be installed before running the build.</code></p>
  <p>I have installed pybind11 using <code>python3 -m pip install pybind</code> and <code>python3 -m pip install &quot;pybind11[global]&quot;</code>. I can even import pybind11 in the python shell. Here is my list of packages:</p>
  <pre><code>Package         Version
  --------------- -------
  cycler          0.10.0
  Cython          0.29.21
  future          0.18.2
  kiwisolver      1.3.1
  matplotlib      3.3.4
  numpy           1.20.1
  Pillow          8.1.0
  pip             21.0.1
  pybind11        2.6.2
  pybind11-global 2.6.2
  pyparsing       2.4.7
  pyserial        3.5
  python-dateutil 2.8.1
  scikit-rf       0.16.0
  setuptools      53.0.0
  six             1.15.0
  smbus2          0.4.1
  </code></pre>