docs: use conda for RTD

Add GDAL to our ReadTheDocs build. Adds a RTD environment file to use in
ReadTheDocs because you can't combine conda and pip deps in the
readthedocs config.

Fixes #207.

.readthedocs.environment.yml

@@ -0,0 +1,29 @@
+name: stactools
+channels:
+  - conda-forge
+  - defaults
+dependencies:
+  - gdal
+  - pip
+  - rasterio~=1.2
+  - pip:
+    - codespell
+    - coverage
+    - flake8
+    - ipython
+    - jupyter
+    - lxml-stubs
+    - mypy
+    - nbsphinx
+    - pylint
+    - sphinx
+    - sphinx-autobuild
+    - sphinx-click
+    - sphinxcontrib-fulltoc
+    - sphinxcontrib-napoleon
+    - types-certifi
+    - types-orjson
+    - types-python-dateutil
+    - types-requests
+    - yapf
+    - .

.readthedocs.yml

@@ -13,9 +13,10 @@ sphinx:
 formats:
   - pdf
 
-python:
-  version: 3.8
-  install:
-    - method: pip
-      path: .
-    - requirements: requirements-dev.txt
+build:
+  os: ubuntu-20.04
+  tools:
+    python: mambaforge-4.10
+
+conda:
+  environment: .readthedocs.environment.yml