Using Anaconda as embedded Python Interpreter does not work on Windows without setting PYTHONHOME

[pscff]

Testing the Windows build in #150 (comment) fails for several tests related to the embedding of the Anaconda Python interpreter

The following tests FAILED:
	 26 - test_storage (Failed)
	 38 - test_mapping (Failed)
	 39 - test_pyembed (Failed)
	 40 - test_python_mapping (Failed)
	 49 - test_json_storage (Failed)
	 52 - test_yaml_storage (Failed)

Windows Anaconda Python version used:

$ python
Python 3.8.5 (default, Sep  3 2020, 21:29:08) [MSC v.1916 64 bit (AMD64)] :: Anaconda, Inc. on win32
Type "help", "copyright", "credits" or "license" for more information.

The typical error is

Fatal Python error: init_fs_encoding: failed to get the Python codec of the filesystem encoding
52: Python runtime state: core initialized
52: ModuleNotFoundError: No module named 'encodings'

This is an Anaconda issue:

• Anaconda3 2019-10 can't be used as embedded interpreter on Windows ContinuumIO/anaconda-issues#11374
• Windows 10 pybind11 embed, Fatal Python error: initfsencoding: unable to load the file system codec pybind/pybind11#1930
• Windows 10: Enable Python Embedding Without Setting PYTHONPATH and PYTHONHOME pybind/pybind11#2369

A temporary workaround is to explicitly set PYTHONHOME in the test environment. This works for the base as well as for interpreters installed in a virtual environment.

[pscff]

The fix makes test 39 "test_pyembed" pass on Windows.

The following tests are still failing:

The following tests FAILED:
         26 - test_storage (SEGFAULT)
         38 - test_mapping (Failed)
         40 - test_python_mapping (SEGFAULT)
         49 - test_json_storage (SEGFAULT)
         52 - test_yaml_storage (SEGFAULT)

[pscff]

The failing tests are fixed in the windows-compile branch (with the exception of test 38 test_mapping)