diff --git a/.travis.yml b/.travis.yml
index 89999e04..855b565c 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -39,7 +39,7 @@ jobs:
         - PYTHON=C:\\Python36\\python
 
     - &linux_s390x_36
-      name: Linux | s390x | Python 365
+      name: Linux | s390x | Python 3.6
       language: python
       python: 3.6
       services: docker
diff --git a/cibuildwheel/bashlex_eval.py b/cibuildwheel/bashlex_eval.py
index 3101089d..0128cfc7 100644
--- a/cibuildwheel/bashlex_eval.py
+++ b/cibuildwheel/bashlex_eval.py
@@ -1,7 +1,7 @@
 import shlex
 import subprocess
 
-from typing import Dict, List, NamedTuple, Optional
+from typing import Dict, NamedTuple
 
 import bashlex  # type: ignore
 
@@ -45,7 +45,7 @@ def evaluate_word_node(node: bashlex.ast.node, context: NodeExecutionContext) ->
     word_start = node.pos[0]
     word_end = node.pos[1]
     word_string = context.input[word_start:word_end]
-    letters: List[Optional[str]] = list(word_string)
+    letters = list(word_string)
 
     for part in node.parts:
         part_start = part.pos[0] - word_start
@@ -53,12 +53,12 @@ def evaluate_word_node(node: bashlex.ast.node, context: NodeExecutionContext) ->
 
         # Set all the characters in the part to None
         for i in range(part_start, part_end):
-            letters[i] = None
+            letters[i] = ''
 
         letters[part_start] = evaluate_node(part, context=context)
 
     # remove the None letters and concat
-    value = ''.join(l for l in letters if l is not None)
+    value = ''.join(letters)
 
     # apply bash-like quotes/whitespace treatment
     return ' '.join(word.strip() for word in shlex.split(value))
diff --git a/cibuildwheel/util.py b/cibuildwheel/util.py
index c466af3f..ecb596b1 100644
--- a/cibuildwheel/util.py
+++ b/cibuildwheel/util.py
@@ -3,7 +3,7 @@ import urllib.request
 from fnmatch import fnmatch
 from time import sleep
 
-from typing import Dict, List, NamedTuple, Optional, Type, TypeVar
+from typing import Dict, List, NamedTuple, Optional
 
 from .environment import ParsedEnvironment
 
@@ -82,17 +82,14 @@ def download(url: str, dest: str) -> None:
         response.close()
 
 
-DependencyConstraints_T = TypeVar('DependencyConstraints_T', bound='DependencyConstraints')
-
-
 class DependencyConstraints:
     def __init__(self, base_file_path: str):
         assert os.path.exists(base_file_path)
         self.base_file_path = os.path.abspath(base_file_path)
 
-    @classmethod
-    def with_defaults(cls: Type[DependencyConstraints_T]) -> DependencyConstraints_T:
-        return cls(
+    @staticmethod
+    def with_defaults() -> 'DependencyConstraints':
+        return DependencyConstraints(
             base_file_path=os.path.join(os.path.dirname(__file__), 'resources', 'constraints.txt')
         )
 
diff --git a/cibuildwheel/windows.py b/cibuildwheel/windows.py
index 4e99094b..89d01a6e 100644
--- a/cibuildwheel/windows.py
+++ b/cibuildwheel/windows.py
@@ -82,9 +82,8 @@ def install_cpython(version: str, arch: str, nuget: str) -> str:
     return installation_path
 
 
-def install_pypy(version: str, arch: str, url: Optional[str]) -> str:
+def install_pypy(version: str, arch: str, url: str) -> str:
     assert arch == '32'
-    assert url is not None
     # Inside the PyPy zip file is a directory with the same name
     zip_filename = url.rsplit('/', 1)[-1]
     installation_path = os.path.join('C:\\cibw', os.path.splitext(zip_filename)[0])
@@ -106,6 +105,7 @@ def setup_python(python_configuration: PythonConfiguration, dependency_constrain
     if python_configuration.identifier.startswith('cp'):
         installation_path = install_cpython(python_configuration.version, python_configuration.arch, nuget)
     elif python_configuration.identifier.startswith('pp'):
+        assert python_configuration.url is not None
         installation_path = install_pypy(python_configuration.version, python_configuration.arch, python_configuration.url)
     else:
         raise ValueError("Unknown Python implementation")