Limit iminuit to versions < 2

Iminuit's developers released version 2 late 2020, and it's a major
overhaul from the previous version 1. It's a major change that will
require major change inside PyPWA to support. In the meantime I have
the setup.py set to exclude version 2.

setup.py

@@ -87,7 +87,7 @@
     "install_requires": [
         "cython",      # C/C++ Optimizations
         "tqdm",        # Progress Bars
-        "iminuit",     # Default minimizer
+        "iminuit<2",     # Default minimizer
         "scipy",       # Needed for Nestle with multiple ellipsoids.
         "numpy",       # Arrays and optimizations
         "pyyaml",      # YAML Parser