Run lint jobs against the source

Note we still need to install beam because isort uses the site-packages to determine third-party libraries.

sdks/python/tox.ini

@@ -28,7 +28,8 @@ select = E3
 
 # Shared environment options.
 [testenv]
-# Ensure that we use the beam installed into our virtual env
+# Ensure that we use the beam installed into our virtual env. This can be
+# overridden for envs that work directly on the source (see lint jobs)
 changedir = {envsitepackagesdir}
 args_are_paths = false
 # Set [] options for pip installation of apache-beam tarball.
@@ -279,6 +280,8 @@ commands =
 # Checks for py2 syntax errors
 deps =
  flake8==3.5.0
+# Run against the source directory.
+changedir = {toxinidir}
 commands =
  time {toxinidir}/scripts/run_mini_py2lint.sh
 
@@ -289,6 +292,8 @@ deps =
  pylint==1.9.3
  isort==4.2.15
  flake8==3.5.0
+# Run against the source directory.
+changedir = {toxinidir}
 commands =
  pylint --version
  time {toxinidir}/scripts/run_pylint_2to3.sh
@@ -300,6 +305,8 @@ deps =
  pylint==2.4.2
  isort==4.2.15
  flake8==3.5.0
+# Run against the source directory.
+changedir = {toxinidir}
 commands =
  pylint --version
  time {toxinidir}/scripts/run_pylint.sh