Fix black ignoring files on travis.

First, black already parsers .gitignore, so no need for the exclude
option in pyproject.toml (apart from checked in generated files). The
problem with this is that .gitignore has `build/` in it already and
black does not take into account the root of the git repository, but
instead seems to assume the current working directory. Thus work around
this by setting the cwd in eachdist.py.

pyproject.toml

@@ -3,16 +3,7 @@ line-length = 79
 exclude = '''
 (
   /(
-      \.eggs         # exclude a few common directories in the
-    | \.git          # root of the project
-    | \.hg
-    | \.mypy_cache
-    | \.tox
-    | \.venv.*
-    | venv.*
-    | \.vscode
-    | dist
-    | ext/opentelemetry-ext-jaeger/src/opentelemetry/ext/jaeger/gen     # generated files
+    ext/opentelemetry-ext-jaeger/src/opentelemetry/ext/jaeger/gen     # generated files
   )/
 )
 '''

scripts/eachdist.py

@@ -308,14 +308,15 @@ def lint_args(args):
 
     runsubprocess(
         args.dry_run,
-        ("black", rootdir)
-        + (("--diff", "--check") if args.check_only else ()),
+        ("black", ".") + (("--diff", "--check") if args.check_only else ()),
+        cwd=rootdir,
         check=True,
     )
     runsubprocess(
         args.dry_run,
-        ("isort", "--recursive", rootdir)
+        ("isort", "--recursive", ".")
         + (("--diff", "--check-only") if args.check_only else ()),
+        cwd=rootdir,
         check=True,
     )
     runsubprocess(args.dry_run, ("flake8", rootdir), check=True)