diff --git a/.travis.yml b/.travis.yml
index f4597719..683004be 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -26,15 +26,19 @@ before_install:
     fi
 
 script:
-  - python3 -m pip install cibuildwheel==1.3.0
+  - python3 -m pip install cibuildwheel==1.6.4
   - python3 -m cibuildwheel --output-dir dist
 
 after_success:
   - |
-    if [[ $TRAVIS_PULL_REQUEST == 'false' ]]; then
+    if [[ $TRAVIS_PULL_REQUEST == "false" && ( $TRAVIS_BRANCH == "master" || $TRAVIS_BRANCH == "develop" ) ]]; then
       python3 -m pip install twine
       python3 -m twine upload --verbose --skip-existing --repository-url https://test.pypi.org/legacy/ dist/*
-      if [[ $TRAVIS_TAG ]]; then python3 -m twine upload --verbose --skip-existing dist/*; fi
+    fi
+  - |
+    if [[ $TRAVIS_TAG ]]; then
+      python3 -m pip install twine
+      python3 -m twine upload --verbose --skip-existing dist/*
     fi
 
 notifications:
@@ -68,6 +72,9 @@ jobs:
     - name: "Linux py38"
       services: docker
       env: CIBW_BUILD="cp38-*"
+    - name: "Linux py39"
+      services: docker
+      env: CIBW_BUILD="cp39-*"
 
     - name: "MacOS py35"
       os: osx
@@ -85,6 +92,10 @@ jobs:
       os: osx
       language: shell
       env: CIBW_BUILD="cp38-*"
+    - name: "MacOS py39"
+      os: osx
+      language: shell
+      env: CIBW_BUILD="cp39-*"
 
     - name: "Win py35"
       os: windows
@@ -102,3 +113,7 @@ jobs:
       os: windows
       language: shell
       env: CIBW_BUILD="cp38-*"
+    - name: "Win py39"
+      os: windows
+      language: shell
+      env: CIBW_BUILD="cp39-*"
diff --git a/gstools/covmodel/base.py b/gstools/covmodel/base.py
index 16ff4845..cbaa1d30 100644
--- a/gstools/covmodel/base.py
+++ b/gstools/covmodel/base.py
@@ -1219,9 +1219,7 @@ def name(self):
     @property
     def do_rotation(self):
         """:any:`bool`: State if a rotation is performed."""
-        return (
-            not np.all(np.isclose(self.angles, 0.0))
-        )
+        return not np.all(np.isclose(self.angles, 0.0))
 
     @property
     def is_isotropic(self):
diff --git a/gstools/krige/base.py b/gstools/krige/base.py
index 8f04d83b..9c3560bd 100755
--- a/gstools/krige/base.py
+++ b/gstools/krige/base.py
@@ -511,13 +511,13 @@ def cond_val(self):
     @property
     def cond_err(self):
         """:class:`list`: The measurement errors at the condition points."""
-        if self._cond_err == "nugget":
+        if isinstance(self._cond_err, str) and self._cond_err == "nugget":
             return self.model.nugget
         return self._cond_err
 
     @cond_err.setter
     def cond_err(self, value):
-        if value == "nugget":
+        if isinstance(value, str) and value == "nugget":
             self._cond_err = value
         else:
             if self.exact:
@@ -634,7 +634,9 @@ def __repr__(self):
         """Return String representation."""
         return (
             "{0}(model={1}, cond_no={2}".format(
-                self.name, self.model.name, self.cond_no,
+                self.name,
+                self.model.name,
+                self.cond_no,
             )
             + ")"
         )
diff --git a/setup.py b/setup.py
index eadc302c..4213ddc5 100644
--- a/setup.py
+++ b/setup.py
@@ -220,6 +220,7 @@ class MPDistribution(Distribution):
     "Programming Language :: Python :: 3.6",
     "Programming Language :: Python :: 3.7",
     "Programming Language :: Python :: 3.8",
+    "Programming Language :: Python :: 3.9",
     "Programming Language :: Python :: 3 :: Only",
     "Topic :: Scientific/Engineering",
     "Topic :: Utilities",