diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index a9ec5263c..7085ac76b 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [ '3.6', '3.7', '3.8' ]
+        python-version: [ '3.7', '3.8', '3.9' ]
     name: Python ${{ matrix.python-version }}
     steps:
     - uses: actions/checkout@v1
diff --git a/.github/workflows/docbuild.yml b/.github/workflows/docbuild.yml
index f1863b6b6..b6a586991 100644
--- a/.github/workflows/docbuild.yml
+++ b/.github/workflows/docbuild.yml
@@ -11,10 +11,10 @@ jobs:
     name: ${{ matrix.build-type }}
     steps:
     - uses: actions/checkout@v1
-    - name: Set up Python 3.8
+    - name: Set up Python 3.9
       uses: actions/setup-python@v1
       with:
-        python-version: 3.8
+        python-version: "3.9"
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
diff --git a/.github/workflows/lint.yml b/.github/workflows/lint.yml
index ca5e6f2b5..01750135b 100644
--- a/.github/workflows/lint.yml
+++ b/.github/workflows/lint.yml
@@ -8,10 +8,10 @@ jobs:
     name: black-flake8
     steps:
     - uses: actions/checkout@v1
-    - name: Set up Python 3.8
+    - name: Set up Python 3.9
       uses: actions/setup-python@v1
       with:
-        python-version: 3.8
+        python-version: "3.9"
     - name: Check formatting with black
       run: |
         pip install black
diff --git a/altair/sphinxext/altairgallery.py b/altair/sphinxext/altairgallery.py
index 1a064c0b3..28393306b 100644
--- a/altair/sphinxext/altairgallery.py
+++ b/altair/sphinxext/altairgallery.py
@@ -303,7 +303,7 @@ def main(app):
 
 def setup(app):
     app.connect("builder-inited", main)
-    app.add_stylesheet("altair-gallery.css")
+    app.add_css_file("altair-gallery.css")
     app.add_config_value("altair_gallery_dir", "gallery", "env")
     app.add_config_value("altair_gallery_ref", "example-gallery", "env")
     app.add_config_value("altair_gallery_title", "Example Gallery", "env")
diff --git a/altair/sphinxext/schematable.py b/altair/sphinxext/schematable.py
index f7f12acc2..880e7e190 100644
--- a/altair/sphinxext/schematable.py
+++ b/altair/sphinxext/schematable.py
@@ -37,7 +37,7 @@ def type_description(schema):
 
 
 def prepare_table_header(titles, widths):
-    """Build docutil empty table """
+    """Build docutil empty table"""
     ncols = len(titles)
     assert len(widths) == ncols
 
@@ -60,7 +60,7 @@ def prepare_table_header(titles, widths):
 
 
 def add_class_def(node, classDef):
-    """Add reference on classDef to node """
+    """Add reference on classDef to node"""
 
     ref = addnodes.pending_xref(
         reftarget=classDef,
@@ -79,7 +79,7 @@ def add_class_def(node, classDef):
 
 
 def add_text(node, text):
-    """Add text with inline code to node """
+    """Add text with inline code to node"""
     is_text = True
     for part in reCode.split(text):
         if part:
diff --git a/doc/conf.py b/doc/conf.py
index 3a39289f3..336d55d0d 100644
--- a/doc/conf.py
+++ b/doc/conf.py
@@ -163,8 +163,8 @@
 # and
 # https://github.com/rtfd/sphinx_rtd_theme/issues/117
 def setup(app):
-    app.add_stylesheet("theme_overrides.css")
-    app.add_stylesheet("custom.css")
+    app.add_css_file("theme_overrides.css")
+    app.add_css_file("custom.css")
 
 # Add any extra paths that contain custom files (such as robots.txt or
 # .htaccess) here, relative to this directory. These files are copied
diff --git a/requirements.txt b/requirements.txt
index de0b5af5e..2e5c2cb59 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,6 +1,6 @@
 entrypoints
 jinja2
-jsonschema
+jsonschema<4.0
 numpy
 pandas>=0.18
 toolz
diff --git a/setup.py b/setup.py
index 303d5bada..297d6187e 100644
--- a/setup.py
+++ b/setup.py
@@ -69,7 +69,7 @@ def find_packages(top=HERE):
     packages=find_packages(),
     include_package_data=True,
     install_requires=get_install_requirements("requirements.txt"),
-    python_requires=">=3.6",
+    python_requires=">=3.7",
     extras_require={"dev": get_install_requirements("requirements_dev.txt")},
     classifiers=[
         "Development Status :: 5 - Production/Stable",
@@ -77,8 +77,8 @@ def find_packages(top=HERE):
         "Intended Audience :: Science/Research",
         "License :: OSI Approved :: BSD License",
         "Natural Language :: English",
-        "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
+        "Programming Language :: Python :: 3.9",
     ],
 )