Update pytest in requirements.txt from 3.9.2 to 4.0.1

[dependencies]

Dependencies.io has updated pytest (a pypi dependency in requirements.txt) from "3.9.2" to "4.0.1".

4.0.1

Bug Fixes

• #3952: Display
  warnings before "short test summary info" again, but still later
  warnings in the end.
• #4386: Handle
  uninitialized exceptioninfo in repr/str.
• #4393: Do not
  create .gitignore/README.md files in existing cache directories.
• #4400:
  Rearrange warning handling for the yield test errors so the opt-out
  in 4.0.x correctly works.
• #4405: Fix
  collection of testpaths with --pyargs.
• #4412: Fix
  assertion rewriting involving Starred + side-effects.
• #4425: Ensure
  we resolve the absolute path when the given --basetemp is a
  relative path.

Trivial/Internal Changes

• #4315: Use
  pkg_resources.parse_version instead of LooseVersion in
  minversion check.
• #4440: Adjust
  the stack level of some internal pytest warnings.