Require Python 3.9 or higher (#743)

Python 3.8 reached end of life October 2024:
https://endoflife.date/python

It's impossible to lock pyproj for all versions 3.9-3.13. So let's split
this into two PRs: one to remove 3.8 support and another to add tests
for 3.12 and 3.13

Author: kylebarron

.github/workflows/test.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.8, 3.9, "3.10", "3.11"]
+        python-version: [3.9, "3.10", "3.11"]
 
     steps:
       - uses: actions/checkout@v4

pyproject.toml

@@ -5,7 +5,7 @@ description = "Fast, interactive geospatial data visualization in Jupyter."
 authors = [{ name = "Kyle Barron", email = "kyle@developmentseed.org" }]
 license = "MIT"
 readme = "README.md"
-requires-python = ">=3.8"
+requires-python = ">=3.9"
 dependencies = [
     "anywidget~=0.9.0",
     # Version in Colab as of October 2024