Pin style checkers and formatters (#295)

Leaving them unpinned, particularly when we have so many now is going to
be a nightmare. Better to have them pinned in the environment.yml and
env/requirements-style.txt files and we upgrade them from time to time.
Fixes a false-positive from the updated RST linter.

env/requirements-style.txt

@@ -1,13 +1,13 @@
 # Requirements for checking code style
 pathspec
-black
-isort
-flake8
-flake8-bugbear
-flake8-builtins
-flake8-functions
-flake8-mutable
-flake8-rst-docstrings
-flake8-simplify
-flake8-unused-arguments
-pep8-naming
+black==21.12b0
+isort==5.10.*
+flake8==4.0.*
+flake8-bugbear==21.11.*
+flake8-builtins==1.5.*
+flake8-functions==0.0.6
+flake8-mutable==1.2.*
+flake8-rst-docstrings==0.2.*
+flake8-simplify==0.14.*
+flake8-unused-arguments==0.0.9
+pep8-naming==0.12.*

environment.yml

@@ -26,17 +26,17 @@ dependencies:
   - sphinx-book-theme==0.0.41
   - sphinx-gallery==0.8.*
   # Code style checks and autoformat
-  - black>=20.8b1
-  - isort
-  - flake8
-  - flake8-bugbear
-  - flake8-builtins
-  - flake8-functions
-  - flake8-mutable
-  - flake8-rst-docstrings
-  - flake8-simplify
-  - pep8-naming
+  - black==21.12b0
+  - isort==5.10.*
+  - flake8==4.0.*
+  - flake8-bugbear==21.11.*
+  - flake8-builtins==1.5.*
+  - flake8-functions==0.0.6
+  - flake8-mutable==1.2.*
+  - flake8-rst-docstrings==0.2.*
+  - flake8-simplify==0.14.*
+  - pep8-naming==0.12.*
   - pip:
       # Install flake8-unused-arguments through pip
       # (not available through conda yet)
-      - flake8-unused-arguments
+      - flake8-unused-arguments==0.0.9

examples/forward/tesseroid.py

@@ -14,7 +14,8 @@
 the downward component of the gravitational acceleration generated by a single
 tesseroid on a computation grid through the :func:`harmonica.tesseroid_gravity`
 function.
-"""
+
+"""  # noqa: RST399
 import boule as bl
 import cartopy.crs as ccrs
 import matplotlib.pyplot as plt