[Snyk] Fix for 4 vulnerabilities

[snyk-bot]

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

``` requests 2.21.0 has requirement urllib3<1.25,>=1.21.1, but you have urllib3 1.25.9. ```

Vulnerabilities that will be fixed

By pinning:

Severity	Issue	Upgrade	Breaking Change	Exploit Maturity
	Authentication Bypass SNYK-PYTHON-CRYPTOGRAPHY-42164	cryptography: 2.1.4 -> 2.3	No	No Known Exploit
	Arbitrary Command Execution SNYK-PYTHON-PYXDG-174562	pyxdg: 0.25 -> 0.26	No	No Known Exploit
	SQL Injection SNYK-PYTHON-SQLALCHEMY-173678	SQLAlchemy: 1.1.15 -> 1.2.18	No	No Known Exploit
	HTTP Header Injection SNYK-PYTHON-URLLIB3-1014645	urllib3: 1.24.3 -> 1.25.9	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.

---

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[nikola-rados]

pyxdg is a part of distutils and thus cannot be uninstalled and reinstalled with certainty. Since none of the vulns are breaking we can close this PR.