Pin pywin32 version

Pywin32 was release on Nov. 10 and introduced an issue when running
virtualenv on windows, mhammond/pywin32#1439. This causes our ci system
to fail when trying to run the windows jobs because tox is uses
virtualenv underneath. This commit adds a pin via the constraints file
to ensure that we are installing the older version which does not have
this problem.

azure-pipelines.yml

@@ -60,7 +60,7 @@ jobs:
           displayName: Install Anaconda packages
         - script: |
             call activate qiskit-ignis
-            python -m pip install --upgrade pip virtualenv
-            pip install tox
+            python -m pip install -c constraints.txt --upgrade pip virtualenv
+            pip install -c constraints.txt tox
             tox -e%TOXENV%
           displayName: 'Install dependencies and run tests'

constraints.txt

@@ -1,2 +1,3 @@
 pylint==2.4.2
 astroid==2.3.1
+pywin32==225

tox.ini

@@ -14,8 +14,8 @@ deps = numpy>=1.13
        Cython>=0.27.1
        setuptools>=40.1.0
 commands =
-    pip install -U git+https://github.com/Qiskit/qiskit-terra.git
-    pip install -U -r{toxinidir}/requirements-dev.txt
+    pip install -U -c constraints.txt git+https://github.com/Qiskit/qiskit-terra.git
+    pip install -U -c constraints.txt -r{toxinidir}/requirements-dev.txt
     stestr run {posargs}
 
 [testenv:lint]