fix(deps): fix main dependencies declaration

Section `[build-system]` is used by `pip` to properly build
package managed by `flit`.
Package's dependencies are taken from section
`[tool.flit.metadata].requires`.
This commit fixes: tonybaloney#142

References:
  + https://flit.pypa.io/en/latest/pyproject_toml.html#build-system-section
  + https://flit.pypa.io/en/latest/pyproject_toml.html#pyproject-old-metadata
  + tonybaloney#142

pyproject.toml

@@ -1,18 +1,6 @@
 [build-system]
-requires = [
-    "flit>=2.0",
-    "gitpython>=3.0.0,<4.0.0",
-    "radon>=4.0.0,<4.1.0",
-    "click>=7.0,<9.0",
-    "nbformat>=5.1.3,<6.0.0",
-    "colorlog>=4.0.0,<5.0.0",
-    "tabulate>=0.8.2,<1.0.0",
-    "plotly>=4.0.0,<5.0.0",
-    "sphinx",
-    "sphinx-click",
-    "progress>=1.4,<2.0",
-    "dataclasses; python_version == '3.6'",]
-build-backend = "flit.buildapi"
+requires = ["flit_core>=2,<4"]
+build-backend = "flit_core.buildapi"
 
 [tool.flit.metadata]
 module = "wily"
@@ -30,17 +18,18 @@ classifiers = [
     "Programming Language :: Python :: 3.10",
     "Programming Language :: Python :: 3.11",
     "Programming Language :: Python :: 3 :: Only",
-    ]
+]
 requires = [
     "gitpython>=3.0.0,<4.0.0",
     "radon>=4.0.0,<4.1.0",
-    "click>=7.0,<8.0",
+    "click>=7.0,<9.0",
     "nbformat>=5.1.3,<6.0.0",
     "colorlog>=4.0.0,<5.0.0",
     "tabulate>=0.8.2,<1.0.0",
     "plotly>=4.0.0,<5.0.0",
-    "progress==1.4",
-    "dataclasses; python_version == '3.6'",]
+    "progress>=1.4,<2.0",
+    "dataclasses; python_version == '3.6'",
+]
 requires-python = ">=3.6"
 description-file = "README.md"