Add HAS_PYGMENTS lazy tester (Qiskit/qiskit#9944)

* Add `HAS_PYGMENTS` lazy tester

This somehow slipped through my nets when I added all the lazy testers.
We hope to drop support for the custom Pygments lexers in the future,
but in the mean time, this is the correct way to test for things, and
having an extra lazy tester doesn't hurt us.

* Remove unused import

qiskit_algorithms/utils/optionals.py

@@ -113,6 +113,11 @@
       - For some graph visualisations, Qiskit uses `pydot <https://github.com/pydot/pydot>`__ as an
         interface to GraphViz (see :data:`HAS_GRAPHVIZ`).
 
+    * - .. py:data:: HAS_PYGMENTS
+      - Pygments is a code highlighter and formatter used by many environments that involve rich
+        display of code blocks, including Sphinx and Jupyter.  Qiskit uses this when producing rich
+        output for these environments.
+
     * - .. py:data:: HAS_PYLATEX
       - Various LaTeX-based visualizations, especially the circuit drawers, need access to the
         `pylatexenc <https://github.com/phfaist/pylatexenc>`__ project to work correctly.
@@ -261,6 +266,7 @@
 HAS_NLOPT = _LazyImportTester("nlopt", name="NLopt Optimizer", install="pip install nlopt")
 HAS_PIL = _LazyImportTester("PIL.Image", name="pillow", install="pip install pillow")
 HAS_PYDOT = _LazyImportTester("pydot", install="pip install pydot")
+HAS_PYGMENTS = _LazyImportTester("pygments", install="pip install pygments")
 HAS_PYLATEX = _LazyImportTester(
     {
         "pylatexenc.latex2text": ("LatexNodes2Text",),