Describe the bug
From this PR, with this failed build.

While adding support for Python 3.10 to https://github.com/googleapis/python-ndb, we see a nonsense result from the summary coverage report: each of the individual unit-x.y jobs reports 100% coverage (including branch coverage), but the summary reports some branches as missed. Part of the nonsense is that if those branches were missed, there would be huge gaps in statement coverage, which aren't being reported.

To Reproduce
How can we reproduce the problem? Please be specific. Don't just link to a failing CI job. Answer the questions below:

1. What version of Python are you using?\

   Python 2.7.18, Python 3.6.15, Python 3.7.12, Python 3.8.12, Python 3.9.7, Python 3.10.0

2. What version of coverage.py are you using? The output of coverage debug sys is helpful.

   $ .nox/cover/bin/coverage debug sys$ .nox/cover/bin/coverage debug sys
   -- sys -------------------------------------------------------
                  coverage_version: 6.0.2
                   coverage_module: .../src/python-ndb/.nox/cover/lib/python3.8/site-packages/coverage/__init__.py
                            tracer: -none-
                           CTracer: available
              plugins.file_tracers: -none-
               plugins.configurers: -none-
         plugins.context_switchers: -none-
                 configs_attempted: .coveragerc
                      configs_read: .../src/python-ndb/.coveragerc
                       config_file: .../src/python-ndb/.coveragerc
                   config_contents: b'[run]\nbranch = True\n\n[report]\nfail_under = 100\nshow_missing = True\nexclude_lines =\n    # Re-enable the standard pragma\n    pragma: NO COVER\n    pragma: NO PY${PY_VERSION} COVER\nomit =\n  */gapic/*.py\n  */proto/*.py\n'
                         data_file: -none-
                            python: 3.8.12 (default, Sep 10 2021, 18:10:10) [GCC 9.3.0]
                          platform: Linux-5.13.0-7614-generic-x86_64-with-glibc2.29
                    implementation: CPython
                        executable: .../src/python-ndb/.nox/cover/bin/python
                      def_encoding: utf-8
                       fs_encoding: utf-8
                               pid: 10362
                               cwd: .../src/python-ndb
                              path: .../src/python-ndb/.nox/cover/bin
                                    /opt/Python-3.8.12/lib/python38.zip
                                    /opt/Python-3.8.12/lib/python3.8
                                   /opt/Python-3.8.12/lib/python3.8/lib-dynload
                                   .../src/python-ndb/.nox/cover/lib/python3.8/site-packages
                      environment: HOME = /home/tseaver
                     command_line: .nox/cover/bin/coverage debug sys
                  sqlite3_version: 2.6.0
           sqlite3_sqlite_version: 3.31.1
               sqlite3_temp_store: 0
          sqlite3_compile_options: COMPILER=gcc-9.3.0; ENABLE_COLUMN_METADATA; ENABLE_DBSTAT_VTAB
                                   ENABLE_FTS3; ENABLE_FTS3_PARENTHESIS; ENABLE_FTS3_TOKENIZER
                                   ENABLE_FTS4; ENABLE_FTS5; ENABLE_JSON1
                                   ENABLE_LOAD_EXTENSION; ENABLE_PREUPDATE_HOOK; ENABLE_RTREE
                                   ENABLE_SESSION; ENABLE_STMTVTAB; ENABLE_UNLOCK_NOTIFY
                                   ENABLE_UPDATE_DELETE_LIMIT; HAVE_ISNAN; LIKE_DOESNT_MATCH_BLOBS
                                   MAX_SCHEMA_RETRY=25; MAX_VARIABLE_NUMBER=250000; OMIT_LOOKASIDE
                                   SECURE_DELETE; SOUNDEX; TEMP_STORE=1
                                   THREADSAFE=1; USE_URI
   

3. What versions of what packages do you have installed? The output of pip freeze is helpful.

   $ .nox/cover/bin/pip freeze
   coverage==6.0.2
   $ .nox/unit-3-10/bin/pip freeze
   attrs==21.2.0
   cachetools==4.2.4
   certifi==2021.10.8
   charset-normalizer==2.0.7
   coverage==6.0.2
   google-api-core==1.31.3
   google-auth==1.35.0
   google-cloud-core==1.7.2
   google-cloud-datastore==1.15.3
   -e git+ssh://[email protected]/googleapis/python-ndb@5cc1001fa78eb9f2aca99c050d1d1518ae3e8f01#egg=google_cloud_ndb
   googleapis-common-protos==1.53.0
   grpcio==1.41.0
   idna==3.3
   iniconfig==1.1.1
   mock==4.0.3
   packaging==21.0
   pluggy==1.0.0
   protobuf==3.17.3
   py==1.10.0
   pyasn1==0.4.8
   pyasn1-modules==0.2.8
   pymemcache==3.5.0
   pyparsing==2.4.7
   pytest==6.2.5
   pytest-cov==3.0.0
   pytz==2021.3
   redis==3.5.3
   requests==2.26.0
   rsa==4.7.2
   six==1.16.0
   toml==0.10.2
   tomli==1.2.1
   urllib3==1.26.7
   

4. What code are you running? Give us a specific commit of a specific repo that we can check out.

   googleapis/python-ndb@5cc1001

5. What commands did you run?

   $ nox -s unit cover
   

Expected behavior

Given that each unit-x.y nox session, including the unit-3.10 one, reports 100% coverage, I
expect that the summary coverage will also show 100% coverage.

Additional context
Add any other context about the problem here.