Adding pip as conda requests.

Signed-off-by: Tim 'mithro' Ansell <me@mith.ro>

environment.yml

diff --git a/environment.yml b/environment.yml
index 41e70a8..c099164 100644
--- a/environment.yml
+++ b/environment.yml
@@ -4,6 +4,7 @@
 dependencies:
   - python
   - symbiflow-yosys
+  - pip
   - pip:
     - tox
     - flake8