Enable ruff's pep8-naming (N) rules

[seisman]

Description of proposed changes
Enable pep8-naming rules to make sure that function/class/variable names follow the PEP8 conventions. This PR fixes the following errors.

ruff check pygmt examples --extend-select=N
pygmt/exceptions.py:38:7: N818 Exception name `GMTInvalidInput` should be named with an Error suffix
   |
38 | class GMTInvalidInput(GMTError):
   |       ^^^^^^^^^^^^^^^ N818
39 |     """
40 |     Raised when the input of a function/method is invalid.
   |

pygmt/exceptions.py:50:7: N818 Exception name `GMTImageComparisonFailure` should be named with an Error suffix
   |
50 | class GMTImageComparisonFailure(AssertionError):
   |       ^^^^^^^^^^^^^^^^^^^^^^^^^ N818
51 |     """
52 |     Raised when a comparison between two images fails.
   |

pygmt/helpers/testing.py:76:5: N806 Variable `ALLOWED_CHARS` in function should be lowercase
   |
74 |     >>> shutil.rmtree(path="tmp_result_images")  # cleanup folder if tests pass
75 |     """
76 |     ALLOWED_CHARS = set(string.digits + string.ascii_letters + "_-[]()")
   |     ^^^^^^^^^^^^^ N806
77 |     KEYWORD_ONLY = inspect.Parameter.KEYWORD_ONLY
   |

pygmt/helpers/testing.py:77:5: N806 Variable `KEYWORD_ONLY` in function should be lowercase
   |
75 |     """
76 |     ALLOWED_CHARS = set(string.digits + string.ascii_letters + "_-[]()")
77 |     KEYWORD_ONLY = inspect.Parameter.KEYWORD_ONLY
   |     ^^^^^^^^^^^^ N806
78 | 
79 |     def decorator(func):
   |

pygmt/src/config.py:9:7: N801 Class name `config` should use CapWords convention 
   |
 9 | class config:
   |       ^^^^^^ N801
10 |     """
11 |     Set GMT defaults globally or locally.
   |

pygmt/src/grdhisteq.py:22:7: N801 Class name `grdhisteq` should use CapWords convention 
   |
22 | class grdhisteq:
   |       ^^^^^^^^^ N801
23 |     r"""
24 |     Perform histogram equalization for a grid.
   |

pygmt/src/triangulate.py:20:7: N801 Class name `triangulate` should use CapWords convention 
   |
20 | class triangulate:
   |       ^^^^^^^^^^^ N801
21 |     """
22 |     Delaunay triangulation or Voronoi partitioning and gridding of Cartesian
   |

Found 7 errors.

[yvonnefroehlich]

Probably N has to be added to the pyproject.toml:

pygmt/pyproject.toml

Lines 101 to 116 in 43f8c63

	select = [
	"B", # flake8-bugbear
	"E", # pycodestyle
	"F", # pyflakes
	"FA", # flake8-future-annotations
	"FLY", # flynt
	"I", # isort
	"ISC", # flake8-implicit-str-concat
	"NPY", # numpy
	"PD", # pandas-vet
	"PIE", # flake8-pie
	"PL", # pylint
	"SIM", # flake8-simplify
	"UP", # pyupgrade
	"W", # pycodestyle warnings
	]

[seisman]

Probably N has to be added to the pyproject.toml:

pygmt/pyproject.toml

Lines 101 to 116 in 43f8c63

	select = [
	"B", # flake8-bugbear
	"E", # pycodestyle
	"F", # pyflakes
	"FA", # flake8-future-annotations
	"FLY", # flynt
	"I", # isort
	"ISC", # flake8-implicit-str-concat
	"NPY", # numpy
	"PD", # pandas-vet
	"PIE", # flake8-pie
	"PL", # pylint
	"SIM", # flake8-simplify
	"UP", # pyupgrade
	"W", # pycodestyle warnings
	]

thanks for catching it. Fixed now.