test: Add lower bound on supported numpy versions (#2087)

Set lower bound on numpy to v1.21.0 as this is the lower bound required by JAX v0.4.1
in addition to the lower bound of numpy>=1.14.5 by scipy v1.5.1 (the current lowest
supported version). Newer versions of scipy place an upper bound on numpy for
compatibility reasons and so this is only required for older scipy releases.

Author: matthewfeickert

tests/constraints.txt

@@ -6,6 +6,7 @@ jsonschema==4.15.0  # c.f. PR #1979
 jsonpatch==1.15
 pyyaml==5.1
 importlib_resources==1.4.0  # c.f. PR #1979
+numpy==1.21.0 # constrained by jax v0.4.1
 # xmlio
 uproot==4.1.1
 # minuit