Fix fitter selection logic in process tomography (qiskit-community#431)

This commit fixes an issue in the process tomography module. When cvxpy
was not installed the module would fail to import. That's because the
selection logic was reworked in qiskit-community#422 but never updated in process
tomography to reflect that change. Since no CI job runs without cvxpy
installed we never caught this edge case. This commit fixes the
underlying issue to rework the process tomography fitter selection logic
to mirror the changes to state tomography in qiskit-community#422 and then also try and
add ci coverage it removes cvxpy from the docs tox job. With warnings
set to fatal this should ensure we are always able to import everything
and build the docs if cvxpy is not installed.

Fixes qiskit-community#429

(cherry picked from commit 01e9070)

qiskit/ignis/verification/tomography/fitters/process_fitter.py

@@ -21,7 +21,7 @@
 from qiskit import QiskitError
 from qiskit.quantum_info.operators import Choi
 from .base_fitter import TomographyFitter
-from .cvx_fit import cvxpy, cvx_fit
+from .cvx_fit import cvx_fit
 from .lstsq_fit import lstsq_fit
 
 
@@ -137,10 +137,11 @@ def fit(self,  # pylint: disable=arguments-differ
                              "to a process matrix.")
         # Choose automatic method
         if method == 'auto':
-            if cvxpy is None:
-                method = 'lstsq'
-            else:
+            self._check_for_sdp_solver()
+            if self._HAS_SDP_SOLVER:
                 method = 'cvx'
+            else:
+                method = 'lstsq'
         if method == 'lstsq':
             return Choi(lstsq_fit(data, basis_matrix, weights=weights,
                                   trace=dim, **kwargs))

requirements-dev.txt

@@ -1,4 +1,3 @@
-cvxpy>=1.0.15
 pylint==2.4.4
 pycodestyle
 qiskit-aer>=0.3.0

tox.ini

@@ -13,6 +13,7 @@ setenv =
 deps = numpy>=1.13
        Cython>=0.27.1
        setuptools>=40.1.0
+       cvxpy>=1.0.15
 commands =
     pip install -U -c constraints.txt qiskit-terra
     pip install -U -c constraints.txt -r{toxinidir}/requirements-dev.txt