[SPARK-34941][PYTHON] Fix mypy errors and enable mypy check for pandas-on-Spark.

[ueshin]

What changes were proposed in this pull request?

Fixes mypy errors and enables mypy check for pandas-on-Spark.

Why are the changes needed?

The mypy check for pandas-on-Spark was disabled when the initial porting.
It should be enabled again; otherwise we will miss type checking errors.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

The enabled mypy check and existing unit tests should pass.

[SparkQA]

Test build #138520 has finished for PR 32540 at commit 626f21e.

• This patch fails Python style tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/43041/

[SparkQA]

Kubernetes integration test status failure
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/43041/

[SparkQA]

Test build #138523 has finished for PR 32540 at commit 1e5a0f3.

• This patch fails Python style tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/43044/

[SparkQA]

Kubernetes integration test status failure
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/43044/

[SparkQA]

Test build #138524 has finished for PR 32540 at commit 607b57a.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[ueshin]

cc @HyukjinKwon @itholic @xinrong-databricks

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/43045/

[SparkQA]

Kubernetes integration test status failure
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/43045/

[SparkQA]

Test build #138526 has finished for PR 32540 at commit 6a0b9c0.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Kubernetes integration test starting
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/43047/

[SparkQA]

Kubernetes integration test status failure
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/43047/

[HyukjinKwon]

Also cc @zero323

[ueshin]

Thanks! merging to master.