Conflicting python package versions since dependabot commit

Running `docker build -t cloudmapper .` as per the README currently yields the following error:

```
ERROR: Cannot install -r requirements.txt (line 5) and urllib3==1.26.5 because these package versions have conflicting dependencies.

The conflict is caused by:
The user requested urllib3==1.26.5
botocore 1.17.6 depends on urllib3<1.26 and >=1.20; python_version != "3.4"

To fix this you could try to:
1. loosen the range of package versions you've specified
2. remove package versions to allow pip attempt to solve the dependency conflict
```

This commit from dependabot appears to have created the problem: https://github.com/duo-labs/cloudmapper/commit/97e0ebcbd7498c3b2beaeeccebc9bbf8171232a1

Editing `requirements.txt` and setting the `urlib` version back fixes the problem: `urllib3==1.25.9`