pyproject.toml: Explicitly specify lnst master branch

Since poetry 1.2.0, if a package has extras and another package depends
on this one twice through other dependencies, but with different extra
requirements, poetry gets stuck and leaks memory. This is a workaround
for that.

See python-poetry/poetry#7194

pyproject.toml

@@ -12,7 +12,7 @@ compare-data-files = "scripts.compare_data_files:main"
 
 [tool.poetry.dependencies]
 python = "^3.9"
-lnst = {git = "https://github.com/LNST-project/lnst"}
+lnst = {git = "https://github.com/LNST-project/lnst", branch = "master"}
 
 [tool.poetry.group.dev]
 optional = true