tests: move mock-ssh-server git dependency (#3953)

This one doesn't allow us to release new pre-releases, because pypi
forbids that:

https://travis-ci.com/github/iterative/dvc/jobs/343343967#L2447

Let's install it separately for now, until there is a new official
mock-ssh-server release.

scripts/ci/install.sh

@@ -10,6 +10,8 @@ scriptdir="$(dirname $0)"
 # work or not.
 $scriptdir/retry.sh pip install .[all,tests]
 
+$scriptdir/retry.sh pip install git+https://github.com/iterative/mock-ssh-server.git
+
 git config --global user.email "[email protected]"
 git config --global user.name "DVC Tester"
 

setup.py

@@ -121,7 +121,7 @@ tests_requirements = [
     "flake8-docstrings",
     "pydocstyle<4.0",
     "jaraco.windows==3.9.2",
-    "mock-ssh-server@git+https://github.com/iterative/mock-ssh-server.git",
+    "mock-ssh-server",
     "moto==1.3.14.dev464",
     "rangehttpserver==1.2.0",
     "beautifulsoup4==4.4.0",