try pytest-mp instead of xdist on win

appveyor.yml

@@ -43,7 +43,8 @@ install:
   - python -c "import sys; print(sys.executable)"
   - python -c "import sys; print(sys.prefix)"
   - conda update -q --all
-  - conda install -q pip pytest pytest-cov jinja2 patch flake8 mock requests contextlib2 chardet glob2 perl pyflakes pycrypto posix m2-git anaconda-client numpy conda-verify beautifulsoup4 pytest-xdist pytest-mock filelock pkginfo psutil
+  - conda install -q pip pytest pytest-cov jinja2 patch flake8 mock requests contextlib2 chardet glob2 perl pyflakes pycrypto posix m2-git anaconda-client numpy conda-verify beautifulsoup4 pytest-mock filelock pkginfo psutil
+  - pip install pytest-mp
   - if "%PYTHON_VERSION%" == "2.7" conda install -q scandir
   # this is to ensure dependencies
   - python --version
@@ -83,10 +84,10 @@ test_script:
   # no libpng here because we already have the right version from the 3.5 install
   - conda create -n blarg -yq --download-only python=3.6
   - conda create -n blarg -yq --download-only cmake
-  - py.test --color=yes -v --cov conda_build --cov-report xml tests --basetemp C:\cbtmp -n 0 -m "serial"
+  - py.test --color=yes -v --cov conda_build --cov-report xml tests --basetemp C:\cbtmp -m "serial"
   # remove all folders to avoid permission errors.  Leave root files (may have coverage info there)
   - for /d %%F in (c:\cbtmp\*) do rd /s /q "%%F"
-  - py.test --color=yes -v --cov conda_build --cov-report xml --cov-append tests --basetemp C:\cbtmp -n 2 -m "not serial"
+  - py.test --color=yes -v --cov conda_build --cov-report xml --cov-append tests --basetemp C:\cbtmp --mp -m "not serial"
 
 # For debugging, this is helpful - zip up the test environment and make it available for later download.
 #    However, it eats up a fair amount of time.  Better to disable until you need it.

conda_build/metadata.py

@@ -208,9 +208,8 @@ def select_lines(data, namespace, variants_in_place):
 
 def yamlize(data):
     try:
-        loader_instance = copy.deepcopy(loader)
-        with stringify_numbers(loader_instance):
-            loaded_data = yaml.load(data, Loader=loader_instance)
+        with stringify_numbers():
+            loaded_data = yaml.load(data, Loader=loader)
         return loaded_data
     except yaml.error.YAMLError as e:
         if '{{' in data:
@@ -797,7 +796,7 @@ def trim_build_only_deps(metadata, requirements_used):
 
 
 @contextlib.contextmanager
-def stringify_numbers(loader):
+def stringify_numbers():
     # ensure that numbers are not interpreted as ints or floats.  That trips up versions
     #     with trailing zeros.
     implicit_resolver_backup = loader.yaml_implicit_resolvers.copy()

setup.cfg

@@ -17,6 +17,7 @@ env =
     PYTHONHASHSEED=0
 markers =
     serial: execute test serially (to avoid race conditions)
+    mp_group_info: for pytest-mp
 
 [versioneer]
 VCS = git