Fix two static type errors reported by mypy 2.0.0 (#4626)

Author: seisman

pygmt/clib/session.py

@@ -110,7 +110,7 @@
 DTYPES = DTYPES_NUMERIC | DTYPES_TEXT
 
 # Dictionary for storing the values of GMT constants.
-GMT_CONSTANTS = {}
+GMT_CONSTANTS: dict[str, int] = {}
 
 # Load the GMT library outside the Session class to avoid repeated loading.
 _libgmt = load_libgmt()

pygmt/figure.py

@@ -93,7 +93,7 @@ def _get_default_display_method() -> Literal["external", "notebook", "none"]:
 
 # A registry of all figures that have had "show" called in this session.
 # This is needed for the sphinx-gallery scraper in pygmt/sphinx_gallery.py
-SHOWED_FIGURES = []
+SHOWED_FIGURES: list["Figure"] = []
 # Configurations for figure display.
 SHOW_CONFIG = {
     "method": _get_default_display_method(),  # The image preview display method.