issues

• [X] Closes #6333
• [X] Tests added
• [X] User visible changes (including notable bug fixes) are documented in whats-new.rst

Adding the import resolves the following exception that sphinx-build raises when one builds only "doc/internals/how-to-add-new-backend.rst":

``` NameError Traceback (most recent call last) Input In [1], in <module> ----> 1 var = xr.Variable( 2 dims=("x",), data=np.arange(10.0), attrs={"scale_factor": 10, "add_offset": 2} 3 )

NameError: name 'xr' is not defined ```

While in the file, I revised headings to have only their first letter capitalized (except for case-sensitive code), which seems to be the majority convention in the documentation.

• [x] User visible changes (including notable bug fixes) are documented in whats-new.rst

This PR enables one to build the documentation on Windows by manually invoking sphinx-build.

The first commit enables sphinx to execute the "conf.py" file. Before the commit, sphinx complains:

Configuration error: There is a programmable error in your configuration file:

with the exception FileNotFoundError: [WinError 2] The system cannot find the file specified where it tries to invoke conda. On Windows, conda environments other than the base environment have "conda.bat" on their path rather than "conda.exe", and one must call subprocess.run with "conda.bat" instead of merely "conda". However, the CONDA_EXE environment variable correctly points to the executable and should do so across platforms; it requires conda version 4.5 or later for conda/conda#6923.

The second commit enables the build to tolerate exceptions when deleting temporary files and tells Git to ignore more such files. Without the commit, builds occasionally failed at the os.remove calls with exceptions such as PermissionError: [WinError 32] The process cannot access the file because it is being used by another process: 'example.nc'.

Should "whats-new.rst" mention these changes, which are developer-visible but not user-visible?

I welcome any feedback.

Edit: Checked the "whats-new" task.

Lengthening the underline resolves the following warning from sphinx-build:

``` [...]\xarray\doc\whats-new.rst:20: WARNING: Title underline too short.

v2022.03.1 (unreleased)

```

• [X] User visible changes (including notable bug fixes) are documented in whats-new.rst

Previously, on Windows, the subject test unnecessarily failed, and the temporary directory and file remained, because the scheduler in the outer context prevented deleting the temporary directory upon exiting the inner context of the latter.

The first commit swaps the two contexts to resolve the issue. The second commit replaces the tempfile.TemporaryDirectory() context manager with pytest's tmp_path fixture, which slightly simplifies the test and relies on pytest to remove the directory on a subsequent invocation.