add support for Python 3.10 (#6422)

* add support for Python 3.10

pyarrow is not available, so we are skipping tests for it.
And, restricting `hdfs` extras to be not installed in Python3.10.

Pathlib removed `_init` method and `init` parameter, so just working
around that. It's not a big change.

* skip windows on 3.10; update pip/wheel/setuptools during install

pywin32 is not yet available for 3.10 Python, so skipping those for now

* install libgit2 with Python3.10 as pygit2 wheel is not available

* install libgit2

* only run on macOS, libgit2-1.1 not available on ubuntu

.github/workflows/tests.yaml

@@ -42,13 +42,14 @@ jobs:
       matrix:
         os: [ubuntu-18.04, windows-2019, macos-10.15]
         pyv: ["3.6", "3.7", "3.8", "3.9"]
-# macos 11.0 workers take too long to start up
-#        include:
-#          - os: "macos-11.0"
-#            pyv: "3.9"
+        include:
+          - os: "macos-10.15"
+            pyv: "3.10.0-rc.1"
         exclude:
           - os: macos-10.15
             pyv: "3.9"
+          - os: macos-10.15
+            pyv: "3.7"
 
     steps:
     - uses: actions/[email protected]
@@ -68,10 +69,14 @@ jobs:
         key: ${{ runner.os }}-pip-${{ hashFiles('setup.py') }}
         restore-keys: |
           ${{ runner.os }}-pip-
+    - name: install libgit2 on Python3.10 in macOS
+      if: matrix.pyv == '3.10.0-rc.1' && matrix.os == 'macos-10.15'
+      run: |
+        brew install libgit2
     - name: install
       if: steps.cache.pip-cache-dir.cache-hit != 'true'
       run: |
-        pip install wheel
+        pip install --upgrade pip setuptools wheel
         pip install -e ".[all,tests]"
     - name: install temporary dependencies
       run: |