Update python_installation.rst

On Python 3.11 and with openblas 0.3.24-2, amici installer is unable to find "cblas.h". This is fixed by exporting the openblas path to BLAS_CFLAGS

documentation/python_installation.rst

@@ -79,6 +79,11 @@ Install the AMICI dependencies via ``pacman``
 
    sudo pacman -S python swig openblas gcc hdf5 boost-libs
 
+Export the bash variable ``BLAS_CFLAGS`` to point to where blas was installed, e.g.:
+.. code-block:: bash
+
+  export BLAS_CFLAGS="-I/usr/include/openblas/"
+
 Install AMICI:
 
 .. code-block:: bash
@@ -99,7 +104,12 @@ Alternatively:
 
    sudo pacman -Su python swig openblas gcc hdf5 boost-libs
 
-3. Install AMICI:
+3. Export the bash variable ``BLAS_CFLAGS`` to point to where blas was installed, e.g.:
+.. code-block:: bash
+
+  export BLAS_CFLAGS="-I/usr/include/openblas/"
+
+4. Install AMICI:
 
 .. code-block:: bash