Minimum python version

Hello my system python is python 3.6, when I installed ford with pip in a venv I got this error.
```
Traceback (most recent call last):
File "/home/i.gianni/libraries/diaglib/.venv/bin/ford", line 8, in
sys.exit(run())
File "/home/i.gianni/libraries/diaglib/.venv/lib64/python3.6/site-packages/ford/__init__.py", line 633, in run
proj_data, proj_docs, md = initialize()
File "/home/i.gianni/libraries/diaglib/.venv/lib64/python3.6/site-packages/ford/__init__.py", line 236, in initialize
return parse_arguments(vars(args), proj_docs, directory)
File "/home/i.gianni/libraries/diaglib/.venv/lib64/python3.6/site-packages/ford/__init__.py", line 513, in parse_arguments
subprocess.run(command, check=True, capture_output=True, text=True)
File "/usr/lib64/python3.6/subprocess.py", line 423, in run
with Popen(*popenargs, **kwargs) as process:
TypeError: __init__() got an unexpected keyword argument 'capture_output'
```
I guessed it was related to a wrong version of python, so a switched to a conda env with python 3.10 and it worked.
Could you please add to the requirements the minimum version of python that works?

Start with the Python package metadata or requirements declarations, then inspect the subprocess.run call in ford/__init__.py that passes capture_output and text. Declare the minimum Python version that supports this usage, and verify that installation on Python 3.6 is rejected while Python 3.10 remains supported.

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