Source demographic data from UN Population API

[SeaCelo]

Update demographics.py to use demographic data from UN Population API

• fertility
• mortality
• population

Also define the base (start) year.

[SeaCelo]

Linking pull request: #7

[SeaCelo]

I'm documenting an issue and fix I ran into when connecting to the UN Population API. On any conda environment except for (base), I was getting an SSL error when trying to download from the API. To replicate from within conda, run these in a python prompt:
python
import pandas as pd
url = 'https://population.un.org/dataportalapi/api/v1/data/indicators/47/locations/710/start/2021/end/2021?format=csv'
df = pd.read_csv(url)
df

If you do this in the base environment, it should work fine. If you switch to another environment it fails with an SSL error:
ssl.SSLError: [SSL: UNSAFE_LEGACY_RENEGOTIATION_DISABLED]

What I think is happening:
The (base) environment uses an older version of openssl (version 1.1.1). When you create a new environment, it installs a newer version (mine was 3.0.7). Openssl version 3 is not fully backward compatible and it seems this newer version doesn't support the older protocols that the UN uses.
The solution is to install the same openssl as (base) on your (ogzaf-dev) environment:
conda install openssl=1.1.1

I will test further to see the latest version of openssl that will still work with the UN site. We may consider adding a version of openssl to the environment.yml file (#15)

@sarvonz Please test this

[SeaCelo]

This is confirmed on the Mac installation and I'm suggesting a change to the installation instructions to require openssl version 1.1.1.

Still need to confirm this on a Windows installation.