DOC: Outdated Python version in contributing docs
#### Location of the documentation
[URL of the documentation](https://pandas.pydata.org/docs/development/contributing.html#creating-a-python-environment-pip)

**Note**: You can check the latest versions of the docs on `master` [here](https://pandas.pydata.org/docs/dev/).
(Problem exist in the latest version: [here](https://pandas.pydata.org/docs/dev/development/contributing.html?highlight=contributing#creating-a-python-environment-pip))
#### Documentation problem

In the documents, it says  `at least Python 3.6.1 installed` to create Python virtual environment to contribute Pandas. However, to install some development requirements (such as NumPy) Python version must be >=3.7. See [here](https://pypi.org/project/numpy/) for NumPy minimum requirement.

#### Suggested fix for documentation
We should update the relevant document so contributors can install an appropriate virtual environment for contribution.
Suggested edit:
 `You’ll need to have at least Python 3.7 installed on your system.`