Add mypy type linting

.coveragerc

@@ -1,3 +1,7 @@
 # .coveragerc
 [report]
-show_missing = True
+show_missing = True
+exclude_lines = 
+    pragma: no cover
+    # ignore lines contains only ellipsis
+    ^\s*\.\.\.\s*$

docs/conf.py

@@ -48,9 +48,9 @@
 master_doc = "index"
 
 # General information about the project.
-project = u"PyFunctional"
-copyright = u"2019, Pedro Rodriguez"
-author = u"Pedro Rodriguez"
+project = "PyFunctional"
+copyright = "2019, Pedro Rodriguez"
+author = "Pedro Rodriguez"
 
 # The version info for the project you're documenting, acts as replacement for
 # |version| and |release|, also used in various other places throughout the
@@ -206,7 +206,7 @@
 
 # -- Options for LaTeX output ---------------------------------------------
 
-latex_elements = {
+latex_elements: dict[str, str] = {
     # The paper size ('letterpaper' or 'a4paper').
     #'papersize': 'letterpaper',
     # The font size ('10pt', '11pt' or '12pt').
@@ -224,8 +224,8 @@
     (
         master_doc,
         "PyFunctional.tex",
-        u"PyFunctional Documentation",
-        u"Pedro Rodriguez",
+        "PyFunctional Documentation",
+        "Pedro Rodriguez",
         "manual",
     ),
 ]
@@ -255,7 +255,7 @@
 
 # One entry per manual page. List of tuples
 # (source start file, name, description, authors, manual section).
-man_pages = [(master_doc, "pyfunctional", u"PyFunctional Documentation", [author], 1)]
+man_pages = [(master_doc, "pyfunctional", "PyFunctional Documentation", [author], 1)]
 
 # If true, show URL addresses after external links.
 # man_show_urls = False
@@ -270,7 +270,7 @@
     (
         master_doc,
         "PyFunctional",
-        u"PyFunctional Documentation",
+        "PyFunctional Documentation",
         author,
         "PyFunctional",
         "One line description of project.",

functional/io.py

@@ -4,6 +4,8 @@
 import io
 import builtins
 
+from typing import Optional
+
 
 WRITE_MODE = "wt"
 
@@ -78,7 +80,7 @@ def read(self):
 
 
 class CompressedFile(ReusableFile):
-    magic_bytes = None
+    magic_bytes: Optional[bytes] = None
 
     # pylint: disable=too-many-instance-attributes
     def __init__(
@@ -280,7 +282,7 @@ def read(self):
 
 
 COMPRESSION_CLASSES = [GZFile, BZ2File, XZFile]
-N_COMPRESSION_CHECK_BYTES = max(len(cls.magic_bytes) for cls in COMPRESSION_CLASSES)
+N_COMPRESSION_CHECK_BYTES = max(len(cls.magic_bytes) for cls in COMPRESSION_CLASSES)  # type: ignore
 
 
 def get_read_function(filename, disable_compression):
@@ -352,7 +354,5 @@ def universal_write_open(
         )
     else:
         raise ValueError(
-            "compression must be None, gz, gzip, lzma, or xz and was {0}".format(
-                compression
-            )
+            f"compression must be None, gz, gzip, lzma, or xz and was {compression}"
         )