Bump pyrsistent from 0.15.1 to 0.15.2

Bumps [pyrsistent](https://github.com/tobgu/pyrsistent) from 0.15.1 to 0.15.2.
- [Release notes](https://github.com/tobgu/pyrsistent/releases)
- [Changelog](https://github.com/tobgu/pyrsistent/blob/master/CHANGES.txt)
- [Commits](tobgu/pyrsistent@v0.15.1...v0.15.2)

Signed-off-by: dependabot[bot] <support@dependabot.com>

requirements.txt

@@ -25,7 +25,7 @@ pyramid==1.10.4
 pyramid-mailer==0.15.1
 pyramid-multiauth==0.9.0
 pyramid-tm==2.2.1
-pyrsistent==0.15.1
+pyrsistent==0.15.2
 python-dateutil==2.8.0
 repoze.sendmail==4.4.1
 requests==2.21.0