Pin mpich<3.4

[valeriupredoi]

Description

• Closes mpich 3.4 (build=external_2) is causing ImportError: libmpi.so.12: cannot open shared object file #2006
• Link to documentation:

---

Before you get started

• ☝ Create an issue to discuss what you are going to do

Checklist

It is the responsibility of the author to make sure the PR is ready to review. The icons indicate whether the item will be subject to the 🛠 Technical or 🧪 Scientific review.

• 🛠 PR has a descriptive title for the changelog
• 🛠 Code follows the style guide
• 🛠 Documentation is available
• 🛠 YAML files pass pre-commit or yamllint checks
• 🛠 Circle/CI tests pass
• 🛠 Codacy code quality checks pass
• 🛠 Documentation builds successfully on readthedocs

New or updated recipe/diagnostic:

• 🛠 Provenance information has been added
• 🛠 New dependencies are added to the project requirements
• 🧪 Documentation for the recipe/diagnostic clearly describes what it calculates from a scientific point of view
• 🧪 Recipe runs successfully on @esmvalbot or some other machine without modification
• 🧪 Figure(s)/data look as expected
• 🧪 Code is well documented and scientifically sound

New or updated data reformatting script:

• 🛠 Dataset is added to the table in the documentation
• 🛠 Documentation contains instructions to obtain the data
• 🛠 Tests for the CMORized data are available
• 🧪 Numbers/units of the data look physically meaningful
• 🛠 Data set is added to the OBS data pool

---

To help with the number pull requests:

• 🙏 We kindly ask you to review two other open pull requests in this repository

[valeriupredoi]

included in #1874 so am gonna close this now