build.yml: move pylint, black, and Sphinx installs to each repo

.github/workflows/build.yml

@@ -34,9 +34,13 @@ jobs:
       with:
         repository: adafruit/actions-ci-circuitpython-libs
         path: actions-ci
-    - name: Install deps
+    - name: Install dependencies
+      # (e.g. - apt-get: gettext, etc; pip: circuitpython-build-tools, requirements.txt; etc.)
       run: |
         source actions-ci/install.sh
+    - name: Pip install pylint, black, & Sphinx
+      run: |
+        pip install pylint black==19.10b0 Sphinx sphinx-rtd-theme
     - name: Library version
       run: git describe --dirty --always --tags
     - name: PyLint