Drop support for Python 3.8 (fatiando#497)

Bump minimal supported Python version to 3.8. Bump minimum versions of
some dependencies (scipy, numba, pandas). Update the table of supported
versions in the docs. Update test workflows to run 3.9 as the oldest
version of Python.

.github/workflows/test.yml

@@ -44,15 +44,15 @@ jobs:
           - optional
         include:
           - dependencies: oldest
-            python: "3.8"
+            python: "3.9"
           - dependencies: latest
             python: "3.11"
           - dependencies: optional
             python: "3.11"
-          # test on macos-13 (x86) using oldest dependencies and python 3.8
+          # test on macos-13 (x86) using oldest dependencies and python 3.9
           - os: macos-13
             dependencies: oldest
-            python: "3.8"
+            python: "3.9"
         exclude:
           # don't test on macos-latest (arm64) with oldest dependencies
           - os: macos-latest

doc/compatibility.rst

@@ -64,3 +64,5 @@ following releases to ensure compatibility:
       - 0.4.0
     * - 3.7
       - 0.6.0
+    * - 3.8
+      - 0.6.0

doc/install.rst

@@ -42,7 +42,7 @@ There are different ways to install Harmonica:
 Which Python?
 -------------
 
-You'll need **Python 3.8 or greater**.
+You'll need **Python 3.9 or greater**.
 See :ref:`python-versions` if you require support for older versions.
 
 Dependencies

pyproject.toml

@@ -22,18 +22,17 @@ classifiers = [
     "Topic :: Scientific/Engineering",
     "Topic :: Software Development :: Libraries",
     "Programming Language :: Python :: 3 :: Only",
-    "Programming Language :: Python :: 3.8",
     "Programming Language :: Python :: 3.9",
     "Programming Language :: Python :: 3.10",
     "Programming Language :: Python :: 3.11",
 ]
-requires-python = ">=3.8"
+requires-python = ">=3.9"
 dependencies = [
     "numpy >= 1.21",
-    "pandas >= 1.1",
-    "scipy >= 1.5",
+    "pandas >= 1.4",
+    "scipy >= 1.9",
     "scikit-learn >= 0.24",
-    "numba >= 0.52",
+    "numba >= 0.53",
     "xarray >= 0.16",
     "verde >= 1.7",
     "xrft >= 1.0",