docs: pin versions for readthedocs

.github/workflows/ci.yml

@@ -111,7 +111,7 @@ jobs:
         key: ${{ runner.os }}-pip-${{ matrix.python }}-${{ matrix.arch }}-${{ hashFiles('tests/requirements.txt') }}
 
     - name: Prepare env
-      run: python -m pip install -r tests/requirements.txt
+      run: python -m pip install -r tests/requirements.txt --prefer-binary
 
     - name: Configure C++11 ${{ matrix.args }}
       shell: bash
@@ -272,7 +272,7 @@ jobs:
       run: python3 -m pip install --upgrade pip
 
     - name: Install dependencies
-      run: python3 -m pip install cmake -r tests/requirements.txt
+      run: python3 -m pip install cmake -r tests/requirements.txt --prefer-binary
 
     - name: Configure
       shell: bash

docs/requirements.txt

-breathe==4.13.1
-sphinx<3
-sphinx_rtd_theme
+breathe==4.20.0
+commonmark==0.9.1
+recommonmark==0.6.0
+sphinx==3.2.1
+sphinx_rtd_theme==0.5.0