Vendor tomli to break the circular dependency.

.github/workflows/test.yml

@@ -26,3 +26,35 @@ jobs:
 
       - name: Codecov upload
         run: codecov
+
+  determine-changes:
+    runs-on: ubuntu-latest
+    outputs:
+      vendoring: ${{ steps.filter.outputs.vendoring }}
+    steps:
+      # For pull requests it's not necessary to checkout the code
+      - uses: dorny/paths-filter@v2
+        id: filter
+        with:
+          filters: |
+            vendoring:
+              # Anything that's touching "vendored code"
+              - "flit_core/flit_core/_vendor/**"
+              - "pyproject.toml"
+        if: github.event_name == 'pull_request'
+
+  vendoring:
+    name: vendoring
+    runs-on: ubuntu-latest
+
+    needs: [determine-changes]
+    if: >-
+      needs.determine-changes.outputs.vendoring == 'true' ||
+      github.event_name != 'pull_request'
+    steps:
+      - uses: actions/checkout@v2
+      - uses: actions/setup-python@v2
+
+      - run: pip install nox
+      - run: nox -s vendoring
+      - run: git diff --exit-code

flit_core/flit_core/_vendor/tomli/LICENSE

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2021 Taneli Hukkinen
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.

flit_core/flit_core/_vendor/tomli/__init__.py

@@ -0,0 +1,9 @@
+"""A lil' TOML parser."""
+
+__all__ = ("loads", "load", "TOMLDecodeError")
+__version__ = "1.2.2"  # DO NOT EDIT THIS LINE MANUALLY. LET bump2version UTILITY DO IT
+
+from flit_core._vendor.tomli._parser import TOMLDecodeError, load, loads
+
+# Pretend this exception was created here.
+TOMLDecodeError.__module__ = "tomli"