Pin matlotlib in CI

PR Qiskit#6087 attempted to fix CI by fixing compatibility with the recent
matplotlib release 3.4.0. However to fix the deprecated API usage we
need new features from matplotlib 3.4.0 which would break compatibility
for users on older matplotlib (as our tutorials job caught since it pins
mpl to avoid a performance regression). As an alternative to that this
commit just pins matplotlib in CI so we do not install the version
emitting deprecation warnings. This means that users will see
deprecation warnings when they use some function (mainly qsphere and
bloch sphere), but until we're ready to raise our minimum supported
matplotlib version to 3.4.0 that is unavoidable.

requirements-dev.txt

@@ -2,7 +2,7 @@ coverage>=4.4.0
 hypothesis>=4.24.3
 ipywidgets>=7.3.0
 jupyter
-matplotlib>=2.1
+matplotlib>=2.1,<3.4
 pillow>=4.2.1
 pycodestyle
 pydot