azure: Install python[23]-devel packages on Cygwin

Install python[23]-devel packages on Cygwin, so that python tests which
require that can run.

(Only the default version of python (currently 3.6) provides the
/usr/lib/pkgconfig/python3.pc file we are looking for so also update to
that version)

azure-pipelines.yml

@@ -122,20 +122,21 @@ jobs:
         librhash0,^
         libuv1,^
         ninja,^
-        python35-pip,^
+        python2-devel,^
+        python3-devel,^
+        python36-pip,^
         vala,^
         wget,^
         cmake,^
         zlib-devel
       displayName: Install Dependencies
     - script: |
-        %CYGWIN_ROOT%\bin\python3.5m.exe -m pip --disable-pip-version-check install pytest-xdist
+        set PATH=%CYGWIN_ROOT%\bin;%SYSTEMROOT%\system32
+        env.exe -- python3 -m pip --disable-pip-version-check install pytest-xdist
       displayName: pip install pytest-xdist
     - script: |
         set BOOST_ROOT=
         set PATH=%CYGWIN_ROOT%\bin;%SYSTEMROOT%\system32
-        # FIXME: we need to support systems without unversioned `python3`
-        cp /usr/bin/python3.5 /usr/bin/python3
         env.exe -- python3 run_tests.py --backend=ninja
       displayName: Run Tests
     - task: CopyFiles@2