STY: Use double quotation marks in pyproject.toml consistently

Use double quotation marks to enclose strings in `pyproject.toml`
consistently.

Author: jhlegarreta

pyproject.toml

@@ -227,26 +227,26 @@ filterwarnings = [
 [tool.coverage.run]
 branch = true
 parallel = true
-concurrency = ['multiprocessing']
+concurrency = ["multiprocessing"]
 omit = [
-  '*/tests/*',
-  '*/testing/*',
-  '*/viz/*',
-  '*/__init__.py',
-  '*/conftest.py',
-  'src/nifreeze/_version.py'
+  "*/tests/*",
+  "*/testing/*",
+  "*/viz/*",
+  "*/__init__.py",
+  "*/conftest.py",
+  "src/nifreeze/_version.py"
 ]
 
 [tool.coverage.report]
 # Regexes for lines to exclude from consideration
 exclude_lines = [
-  'raise NotImplementedError',
-  'warnings\.warn',
+  "raise NotImplementedError",
+  "warnings\\.warn",
 ]
 
 [tool.codespell]
 # nd - import scipy.ndimage as nd
 # mapp, reson -- Mapp. and Reson. abbreviations in citation
-ignore-words-list = 'nd,mapp,reson'
+ignore-words-list = "nd,mapp,reson"
 skip = """
 ./.git,*.pdf,*.svg,*.min.js,*.ipynb"""