[Snyk] Fix for 7 vulnerabilities

Snyk has created this PR to fix one or more vulnerable packages in the `pip` dependencies of this project.

Changes included in this PR

Changes to the following files to upgrade the vulnerable dependencies to a fixed version:
- requirements.txt

⚠️ Warning

``` importlib-metadata 1.7.0 requires zipp, which is not installed. beautifulsoup4 4.9.1 requires soupsieve, which is not installed. beautifulsoup4 4.9.1 requires soupsieve, which is not installed. ```

Vulnerabilities that will be fixed

By pinning:

Issue	Upgrade	Breaking Change	Exploit Maturity
Sandbox Escape SNYK-PYTHON-JINJA2-174126	`jinja2:` `2.10 -> 2.10.1`	No	No Known Exploit
Arbitrary Code Execution SNYK-PYTHON-NUMPY-73513	`numpy:` `1.15.4 -> 1.16.3`	No	No Known Exploit
Double Free SNYK-PYTHON-PSUTIL-483082	`psutil:` `5.4.8 -> 5.6.7`	No	No Known Exploit
Arbitrary Code Execution SNYK-PYTHON-PYYAML-42159	`pyyaml:` `3.13 -> 4.2b1`	No	No Known Exploit
Arbitrary Code Execution SNYK-PYTHON-PYYAML-559098	`pyyaml:` `3.13 -> 4.2b1`	No	No Known Exploit
CRLF injection SNYK-PYTHON-URLLIB3-174323	`urllib3:` `1.24.1 -> 1.24.3`	No	Mature
Improper Certificate Validation SNYK-PYTHON-URLLIB3-174464	`urllib3:` `1.24.1 -> 1.24.3`	No	No Known Exploit

Some vulnerabilities couldn't be fully fixed and so Snyk will still find them when the project is tested again. This may be because the vulnerability existed within more than one direct dependency, but not all of the effected dependencies could be upgraded.

Check the changes in this PR to ensure they won't cause issues with your project.

Note: You are seeing this because you or someone else with access to this repository has authorized Snyk to open fix PRs.

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