Update changelog and fix lint.

Author: dihm

docs/source/changelog.rst

@@ -7,6 +7,12 @@ Latest
 Improvements
 ++++++++++++
 
+- `leveldiagram` and `arc` dependencies are now lazy loaded at first use
+  (`draw_diagram` and `Cell` instantiation, respectively).
+  This improves `import rydiqule as rq` time by about a second
+  and ensures second-order dependencies such as matplotlib and sympy are not
+  imported unless explicitly needed.
+
 
 Bug Fixes
 +++++++++

src/rydiqule/cell.py

@@ -16,7 +16,7 @@
 from .sensor_utils import ScannableParameter, TimeFunc
 from .atom_utils import calc_kappa, calc_eta, expand_qnums, validate_qnums, A_QState, ground_state, _load_arc_atom
 from .arc_utils import RQ_AlkaliAtom
-from .exceptions import RydiquleError, AtomError, CouplingNotAllowedError
+from .exceptions import RydiquleError, CouplingNotAllowedError
 from .exceptions import RydiquleWarning, debug_state
 
 from typing import Literal, Optional, Sequence, List, Tuple, Callable, Union, Dict