[PATCH 2 of 3] requirements: bump pytest minimum version to 3.8.0

[Thomas De Schampheleire]

# HG changeset patch
# User Thomas De Schampheleire <thomas.de_schampheleire at nokia.com>
# Date 1564519459 -7200
#      Tue Jul 30 22:44:19 2019 +0200
# Node ID ec962541951a19860f3e9ed49b0b44d81c2b9a9d
# Parent  d13fbe9d0a9e6b59e11cf8c546da66672b087c96
requirements: bump pytest minimum version to 3.8.0

pip with minimum versions reports:

ERROR: pytest-benchmark 3.2.2 has requirement pytest>=3.8, but you'll have
pytest 3.3.0 which is incompatible.

diff --git a/dev_requirements.txt b/dev_requirements.txt
--- a/dev_requirements.txt
+++ b/dev_requirements.txt
@@ -1,4 +1,4 @@
-pytest >= 3.3.0, < 4.7
+pytest >= 3.8.0, < 4.7
 pytest-runner < 5.2
 pytest-sugar >= 0.7.0, < 0.10
 pytest-benchmark < 3.3