Fix the doc-generating CI by pinning an older Jinja2 version

See [here](sphinx-doc/sphinx#10291) for
details

.cproject

@@ -120,7 +120,7 @@
 									<listOptionValue builtIn="false" value="BUILD_TIME=&quot;&quot;"/>
 									<listOptionValue builtIn="false" value="RELEASE=&quot;&quot;"/>
 								</option>
-								<option IS_BUILTIN_EMPTY="false" IS_VALUE_EMPTY="false" id="gnu.cpp.compiler.option.include.paths.368410987" superClass="gnu.cpp.compiler.option.include.paths" valueType="includePath">
+								<option IS_BUILTIN_EMPTY="false" IS_VALUE_EMPTY="false" id="gnu.cpp.compiler.option.include.paths.368410987" name="Include paths (-I)" superClass="gnu.cpp.compiler.option.include.paths" useByScannerDiscovery="false" valueType="includePath">
 									<listOptionValue builtIn="false" value="&quot;${workspace_loc:/oxDNA/src}&quot;"/>
 								</option>
 								<inputType id="cdt.managedbuild.tool.gnu.cpp.compiler.input.632532692" superClass="cdt.managedbuild.tool.gnu.cpp.compiler.input"/>
@@ -139,7 +139,7 @@
 						</toolChain>
 					</folderInfo>
 					<sourceEntries>
-						<entry excluding="Backends|Observables|src/Backends" flags="VALUE_WORKSPACE_PATH|RESOLVED" kind="sourcePath" name=""/>
+						<entry flags="VALUE_WORKSPACE_PATH" kind="sourcePath" name=""/>
 					</sourceEntries>
 				</configuration>
 			</storageModule>

oxpy/build_docs/requirements.txt

@@ -1,4 +1,5 @@
 Sphinx==3.4.1
 sphinx_rtd_theme==0.5.0
 recommonmark==0.7.1
-docutils==0.16
+docutils==0.16
+Jinja2<3.1