maint: add lxml optional dependency

Author: oesteban

Dockerfile

@@ -111,6 +111,7 @@ RUN conda install -y -c anaconda -c conda-forge \
                      python=3.7 \
                      libxml2=2.9 \
                      libxslt=1.1 \
+                     lxml \
                      mkl \
                      mkl-service \
                      numpy=1.20 \

setup.cfg

@@ -40,13 +40,19 @@ packages = find:
 include_package_data = True
 
 [options.extras_require]
+niftiext =
+    lxml
+niftiexts =
+    %(niftiext)s
 test =
     pytest
     pytest-cov
     codecov
 tests =
     %(test)s
+
 all =
+    %(niftiext)s
     %(test)s
 
 [options.entry_points]