openPMD-api: 0.13.0

Bump the openPMD-api dependency to the latest release.

This updated the `requirements.txt` slightly to use
[semantic versioning](https://www.python.org/dev/peps/pep-0440/#compatible-release) ([SO](https://stackoverflow.com/questions/39590187/in-requirements-txt-what-does-tilde-equals-mean)).

openPMD-api follows [semantic versioning](https://semver.org/),
thus APIs are guaranteed to be compatible for those changes:
- `0.Y.A` -> `0.Y.B`
- `X.A.*` -> `X.B.*`
- `<any>.<any>.*` -> `<any>.<any>.*`

In reality, our APIs expose breaking changes even less often and
would often not even affect this repo (e.g. we also release a C++ API
that is covered under the same SemVer). Nonetheless, I have some
ideas on my backlog that I want to improve in the Python API, a few
of them would introduce breakage
(e.g. openPMD/openPMD-api#808). Updating
the `requirements.txt` file manually with a newer major release once
it is released (or desired) will ensure stability & reproducibility.

Breaking changes are always documented in
[NEWS.rst](https://github.com/openPMD/openPMD-api/blob/dev/NEWS.rst)
that is also embedded in the the
[manual](https://openpmd-api.readthedocs.io/en/latest/install/upgrade.html).
Do not hesitate to @-me, too.

Thanks to the
[GitHub dependency resolver](https://github.com/openPMD/openPMD-api/network/dependents?package_id=UGFja2FnZS0xNzQ2MzEwODY%3D) that parses your `requirements.txt`,
I also see the dependency to this repo, which helps me to keep track
during release workflows, so I can help if need be.

requirements.txt

@@ -1,5 +1,5 @@
 # openPMD_to_gdf.py, gdf_to_openPMD.py
-openpmd-api>=0.12.0,<0.13.0
+openpmd-api~=0.13.0
 # gdf_to_openPMD.py
 matplotlib>=3.0.0
 # OpenPMD_add_patches.py