Pin pyparsing<3 in constraints (#7174)
Pin pyparsing<3 in constraints (#7174)

Matplotlib's LaTeX handling is broken by the new release of pyparsing
3.0.  We are already restricting matplotlib<3.4 in CI due to
deprecations, so we are unlikely to get updated versions of matplotlib
any time soon, which we will likely need to lift the pin.
QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py,QISKit/qiskit-sdk-py