BLD: fix python version in test images

I discovered the source of the Python 3.9 CI failures (along with general issues with pymc.)

I had accidentally pinned the Python version to the first release (e.g., 3.9.0, 3.10.0, ...), see here. The Python 3.9 environment now builds and the tests pass for me, hopefully they also will in our CI.

containers/dockerfile-template

@@ -3,7 +3,7 @@ LABEL name="bilby CI testing" \
 maintainer="Gregory Ashton <gregory.ashton@ligo.org>, Colm Talbot <colm.talbot@ligo.org>"
 
 COPY env-template.yml env.yml
-RUN echo "  - python=={python_major_version}.{python_minor_version}" >> env.yml
+RUN echo "  - python={python_major_version}.{python_minor_version}" >> env.yml
 ENV conda_env python{python_major_version}{python_minor_version}
 
 RUN mamba env create -f env.yml -n ${{conda_env}}